Aquaculture Engineering Surveying Techniques PDF

UNIT 1: SURVEYING Definition: Surveying is the art of determining the relative positions of points on above or beneath the surface of the earth by means of direct or indirect measurement of distance, direction and elevation. It is also includes the art of esta...

UNIT 1: SURVEYING Definition: Surveying is the art of determining the relative positions of points on above or beneath the surface of the earth by means of direct or indirect measurement of distance, direction and elevation. It is also includes the art of establishing points by predetermined angular and linear measurements. The purpose of survey is to prepare plan or map so that it may represent the area on a horizontal plane. Principles of surveying The fundamental principles of survey are based on two aspects. 1) Location of a point by measurement, from two points of references. 2) Working from whole to part. Location of a point by measurement from two points of references The relative positions of the points to be surveyed should be located by measurement from at least two points of reference, the positions of which have been already fixed. Let P and Q be the reference points on the ground. The distance PQ can be measured accurately and the relative positions of P and Q can be plotted on the sheet to some scale, using these reference points other points can be located.  The distance PQ is known, measure the distance QR and PR. The point R can be plotted by swinging the two arcs to the same scale to which PQ has been plotted. This principle is very much used in chain surveying.  A perpendicular line can be dropped on the reference line PQ and the length PS and SR is measured. This principle is used for defining details.  Using the distance QR and the angle PQR, the point R can be plotted. This principle is used in traversing.  Using the angle PQR and RPQ the position of R can be plotted. This principle is very much used in triangulation and this method is used for very extensive work.  Angle PQR and the distance PR can be used for plotting of R. This principle used in traversing is of minor utility. Working from whole to part It is very essential to establish first a system of control points and to fix them with higher precision. Minor control points can be established by less precise methods. The idea of working in this way is to prevent the accumulation of errors and to control and localize minor errors, which otherwise would expand to greater magnitude if the reverse process is followed thus making the work uncontrolled at the end. Classification of surveying Primarily surveying can be divided into two classes Plane surveying: Is the type of surveying in which the mean surface of the earth is considered as a plane surface and the spheroidal shape is neglected. All triangles formed by survey lines are considered as plane triangles. The level lines are considered as straight and all plumb lines are considered as parallel. 1 Geodetic surveying: Is the type of surveying in which the shape of the earth is taken into account. The lines lying on the surface are considered as curved lines. The triangles formed by survey lines are considered as spherical triangles. All Geodetic survey includes the work of large magnitude and high degree of precision. Classification based upon the nature of the field of survey  Land surveying  Marine or Hydrographic surveying  Astronomical surveying Land surveying: Land surveying is subdivided into three types.  Topographical surveying: This consists of horizontal and vertical location of certain points by linear and angular measurement and is made to determine the natural features of a country such as rivers, streams, lakes, hills etc., and such artificial features as roads, railways, canals, towns and villages.  Cadastral surveys: Cadastral surveys are made incident to the fixing of property lines, the calculation of land, area or the transfer of land property from one owner to another. They are also made to fix the boundaries of municipalities, state and federal jurisdictions.  City surveying: This survey is useful in the construction of streets, water supply system, sewers and other works. Marine or Hydrographic survey: This survey deals with bodies of water for purpose of navigation, water supply, harbor works or for the determination of mean sea level. The work consists in measurement of discharge of streams, making topographic survey of shores and banks, taking and locating soundings to determine the depth of water and observing the fluctuations of the ocean tide. Astronomical survey: Astronomical survey is to determining the absolute locations of any point or the absolute location and direction of any line in the surface of the earth. This consists in observations to the heavenly bodies such as the sun or any fixed star. Classification based on the Instruments  Chain surveying  Theodolite surveying  Traverse surveying  Triangulation surveying  Tacheometric surveying  Plane table surveying  Photographic surveying  Aerial surveying Chaining: Chaining is the term, which is used to denote the measuring distance either with the help of chain or a tape and is the most accurate method of making direct measurements. For a work of ordinary precision a chain can be used, but for higher precision a tape or special bar can be used. 2 Instruments for chaining:  Chain or Tape  Cross staff  Ranging rod  Arrows  Pegs  Plumb bob  Field book, pencil etc., Chain: Chains are formed at straight links of galvanized mild steel wire bent into rings at the ends and joined each other by three small circular or oval wire rings. These rings offer flexibility to the chain. The ends of the chain are provided with brass handle at each end with swivel joint, so that the chain can be turned without twisting. The length of a link is the distance between the centers of two consecutive middle rings, while the length of the chain is measured from the outside of one handle to the outside of the other handle. Following are the various types of chain  Metric chain  Surveyor’s or Gunter’s chain  Engineer’s chain  Revenue chain  Steel band or Band chain Types of chain Metric chain Metric chains are commonly used in now a days. Metric chains are generally available in lengths of 5, 10, 20 and 30 m. To enable the reading of fractions of a chain without much difficulty, tallies are fixed at every meter length for chains of 5 and 10 m length. Tallies are provided at every 5 m length, for the chain of 20 and 30 m length. Small brass rings are provided at every meter length. To facilitate holding of arrows in position with the handle of the chain, a groove is cut on the outside surface of the handle. The tallies used for making distances in the metric chains are marked with the letters “m” in order to distinguish them from non metric chains. 3 Gunter’s chain or Surveyors chain A Gunter’s chain or surveyor’s chain consists of 100 links of 66ft long. Each link being 0.66 ft. or 7.92 inches long. It is used for land measurement. Engineer’s chain Engineer’s chain is 100 ft long and consists of 100 links. Each link is being 1 ft long. Revenue chain The Revenue chain is 33 ft long and consists of 16 links, each link being 2 1/16 ft long. This chain is mainly used for measuring fields in cadastral survey. Steel band or Band chain Steel band or Band chain consists of a long narrow strip of blue steel of uniform width of 12 to 16 mm and thickness of 0.3 to 0.6mm. Metric steel bands are available in length of 20 or 30m. During continuous use the length of a chain gets altered. Its length is shortened chiefly due to the bending of the links. Its length is elongated either due to stretching of the links and joints opening out of the small rings or due to the wear of wearing surface. For accurate work it is necessary to test the length of the chain from time to time and make adjustments in the length. Tapes: Tapes are used for more accurate measurements. They are classified according to the material of which they are made.  Cloth tape or Linen tape.  Metallic tape.  Steel tape.  Invar tape. Types of Tapes ClothTape or Linen tape Cloth tapes are made up of closely woven linen, 12 to 15 mm wide, varnished to resist moisture. They are light and flexible, and may be used for taking comparatively rough and subsidiary measurement such as off sets. Cloth tapes are available in length 10, 20, 25 and 30 m (30, 50, 66, 100 ft). The end of the tape is provided with small brass ring whose length is also included in the total length of the tape. It is not used for accurate measurements because  It is easily affected by moisture or dampness  The length gets altered by stretching  It is not strong  It is likely to be twist. Metallic tape A metallic tape is made up of varnished strip of water proof linen inter-woven with small brass, copper or bronze wires and does not stretch as easily as a cloth tape. Due to this reason it is used for some accuracy of work. Metallic tapes are made in length of 2, 5, 10, 20, 25, 30 and 50 m. 4 Steel tape Steel tapes vary in quality and accuracy of graduation, but even a poor steel tape is generally superior to a cloth or metallic tape for most of the linear measurements that are made in surveying. A steel tape consists of a light strip of width 6 to 10 mm and is more accurately graduated. Steel tapes are available in lengths of 1, 2, 10, 20, 30 and 50 m. Invar tape Invar tapes are used mainly for linear measurements of a very high degree of precision, such as measurement of base lines. The invar tape is made of alloy of nickel and steel, and has very low co-efficient of thermal expansion. Invar tapes and bands are more expensive. Invar tapes are normally 6 mm wide and are available in lengths of 20, 30 and 100 m. Ranging rods Ranging rods have a length of 2 or 3 m. It is made up of well seasoned wood or steel conduit. Ranging rods are having circular or octagonal shape with 3 cm cross section. They are shod at the bottom with a heavy iron point. The ranging rods are painted alternatively with two colours (Black and White or Red and White). Ranging rods are used to range some intermediate points in the survey line. These rods are can also be used for rough measurement of short length. Arrows: An arrow is made up of steel wire. It is inserted into the ground after every chain length measured on the ground. It consists of 50 mm loop at top and 400 mm length. Pegs: Wooden pegs are used to mark the positions of the stations or terminal points of survey lines. They are made of stout timber. These are available about 2.5 to 3 cm square and 15 cm length. They are driven in the ground with the help of a wooden hammer and kept about 4 cm projection above the ground. Plumb bob: While chaining along a sloping ground, a plumb bob is required to transfer the points to the ground. It is used as centering aid in the theodolites, compass, plane table and a variety of other survey instruments. Ranging, types of ranging Ranging of survey line: While measuring the length of a ‘Survey line’ or ‘Chain line’, the chain or tape must be stretched straight along the line joining its two terminal stations. If the length of the survey line is less than the length of the chain, there will be no difficulty in doing so. However, if the length of survey line exceeds the length of the chain, some intermediate points will have to be established in line with the two terminal points before chaining is started. 5 The process of fixing or establishing such intermediate points is known as Ranging. Code of Signals for Ranging Methods of Ranging: There are two methods of ranging  Direct ranging  Indirect ranging Direct ranging: Direct ranging is done when the two ends of the survey lines are intervisible. In such cases ranging can either be done by eye or through some optical instruments such as line ranger or a Theodolite. Ranging by eye: Let A and B be the two points at the ends of a survey line. One ranging rod is erected at the point ‘B’ while the surveyor stands with another ranging rod at point ‘A’ holding the rod at about half meter length. The assistant then goes with another ranging rod and establishes the rod at a point approximately in the line with AB, at a distance not greater than one chain length from ‘A’. The surveyor at ‘A’ then signals the assistant to move transverse to the chain line, till he is in line with A and B. Similarly other intermediate points can be established. Indirect ranging or Reciprocal ranging Indirect ranging is resorted to when both the ends of the survey line are not intervisible either due to high intervening ground or due to long distance between them. In such a case, ranging is done indirectly by selecting two intermediate points M1 and N1 very near to the chain line in such a way that from M1 both N1 and B are visible, while from N1 both M1 and A are visible. 6 Two surveyors station themselves at M1 and N1 with ranging rods. The person at M1 then directs the person at N1 to move to a new position N2 in line with M1B. The person at N2 then directs the person at M1 to move to a new position M2 in line with N2 A. Thus the two persons are now at M2 and N2 which are near to the chain line than the positions M1 and N1. This process is repeated till the points M and N are located in such a way that the person at M finds the person at N in line with MB and the person at N finds the person at M in line with NA. After having established M and N other points can be fixed by direct ranging. Chaining – Chaining on uneven or sloping ground Two chain men are required for measuring the length of a line, which is greater than a chain length. The more experienced of the chainmen remains at the zero end or rear end of the chain and is called the follower. The other chainman holding the forward handle is known as the leader. Unfolding the chain To unfold a chain, the chainman keeps both the handles in the left hand and throws the rest of the portion of the chain in the forward direction with his right hand. The other chainman assists in removing the knots, etc., for making the chain straight. Folding the Chain Bring the two halves of the chain so as to lie along each other by pulling the chain in the middle. Commencing from the middle take two pairs of links at a time with the right hand and place them obliquely across with the left hand. Lining and marking The follower holds the zero end of the chain at the terminal point while the leader proceeds forward with the other handle in one hand and a set of 10 arrows and a ranging rod in the other hand. When he is approximately one chain length away, the follower directs him to fix his pole in line with the previous pole. When the point is ranged, the leader makes a mark on the ground, holds the handle with both the hands and pulls the chain so that it becomes straight between the terminal point and the point fixed. The leader then puts an arrow at the end of the chain, and then the leader swings the chain slightly out of the line and proceeds further with the handle in one hand and the rest of the arrows and the ranging rod in the other hand. The follower also takes the handle in one hand and ranging rod in the other hand and follows the leader till the leader has approximately travelled one chain length. The follower puts the zero end of the chain at the first arrow fixed by the leader and ranges the leader who in turn stretches the chain straight in the line and fixes the second arrow in the ground and proceeds further. Chaining on uneven or sloping ground To get the horizontal distance in sloping ground, there are two methods  Direct method  Indirect method 7 Direct method or stepping method In this method the distance measured in small horizontal stretches or steps. The follower holds the zero end of the tape at A, the leader select any suitable length l1 of the tape and moves forward. The follower directs the leader for ranging. The leader pulls the tape tight, make it horizontal and the point 1(one) is transferred to the ground by plumb bob or by using drop arrow. The procedure is repeated. The total length “D” of the line is then equal to l1+l2+l3+…………………… It is more convenient to measure down-hill than to measure uphill. The lengths l1, l2 etc., to be selected depend on the steepness of the slope. Steeper the slope, lesser the length and vice versa. Indirect method (i) In the case of regular slope the sloping distance and sloping angle can be measured and using this horizontal distance can be calculated. L1 be the measured inclined distance between AB. θ1 = Slope of the line AB with horizontal The horizontal distance D1 = L1 Cos θ1 Similarly for BC = D2 = L2 Cos θ2 Total length = D1+D2 (ii) Difference in level Measured The Difference in the level between the points is measured with the help of a levelling instrument and the horizontal distance computed using the below formula. Calculation of the horizontal distance of sloping ground The distance between two points measured along a slope is 428 m. Find the horizontal distance between them if (a) The angle of slope between the points is 8° (b) The difference in level is 62 m (c)The slope is 1 in 4 8 Error due the incorrect chain length If the length of the chain used in measuring length of the line is not equal to the true length or designated length, the measured length of the line will not be correct and suitable correction will have to be applied. If a chain is too long, the measured distance will be less. The error will be negative and the correction is positive. Similarly if a chain is too short, the measured distance will be more, the error will be positive and the correction will be negative.  Correction for measured length. Correction for measured Area Correction for measured volume 9 Chain surveying Definition Chain surveying is the type of surveying in which linear measurements are made in the field. This type of surveying is suitable for surveys of small extent in open ground to secure the data for exact description of the boundaries of piece of land. Principles of Chain surveying The principle of chain survey or chain triangulation is to provide a skeleton or frame work consisting of number of connected triangles, as a triangle is the only simple figure that can be plotted from the lengths of its side measured in the field.  Survey stations: A survey station is the prominent point on the chain line. If it is at the beginning or at the end of the chain line is known as Main survey station.  Survey lines: The lines joining the survey stations are called as survey lines. The lines joining the main survey stations are called as main survey lines. The biggest of the main survey lines is called as base line. 10 Check lines: Check lines or proof lines are the lines which are run in the field to check the accuracy of the work. The length of the check line measured in the field must agree with its length on the plan. A check line may be laid by joining the apex of the triangle to any point on the opposite side or by joining two points on any two sides of a triangle Tie line: Tie line is a line which joins subsidiary or tie stations on the main line. The main object of running a tie line is to take the details of nearby objects but it also serves the purpose of check lines. Instruments used for setting out right angles There are several types of instruments used to set out a right angle to a chain line, the most common being Cross staff, Optical square and Prism square. There are three types of cross staff namely  Open cross staff  French cross staff  Adjustable cross staff Open cross staff It is the simplest instrument used for setting out right angle. It consists of either a frame or box with two pairs of vertical slits and is mounted on a pole shod for fixing in the ground. The two pairs of vertical slits give two lines of sights at right angles to each other. French cross staff It consists of a hollow octagonal box. Vertical sight slits are cut in the middle of each face, such that the lines between the centres of opposite slit makes an angle of 450 with each other. It is possible to set-out angles of either 450 or 900 with this instrument. Setting out right angle by using cross staff The cross staff is set up at a point on the line from which the right angle is to run, and then turned until one line of sight passes through the ranging rod at the end of the survey line. The line of sight through the other two vanes will be a line at right angles to survey line and a ranging rod may be established in that direction. Conditions to be fulfilled by survey stations/survey lines  Survey stations must be mutually visible.  Survey lines must be as few as possible so that the frame work can be plotted conveniently.  The frame work must have one or two base lines. If one base line is used, it must run along the length and through the middle of the area. If two base lines are used, they must intersect in the form of letter ‘X’  The lines must run through as level ground as possible  The main lines should form well-conditioned triangle  Each triangle or portion of skeleton must be provided with sufficient check lines.  As far as possible the main survey lines should not pass through obstacles.  To avoid trespassing, the main survey lines should fall within the boundaries of the property to be surveyed. 11 Off-sets An offset is the lateral distance of an object or ground feature measured from a survey line. By method of offsets, the point or object is located by measurement of a distance and angle from a point on the chain line when the angle of offset is 90º, it is called perpendicular offset or simply offset. When the angle is other than 90º, it is called oblique offset. Another method of locating a point is called the method of “ties” in which the distance of the point is measured from two separate points on the chain line such that the three points form, as nearly as possible an equilateral triangle. Survey may be done in the following steps.  Reconnaissance  Marking and fixing survey station  Running a survey lines Reconnaissance Before starting the actual survey measurements, the surveyor should walk around the area to fix best positions of survey lines and survey stations. During reconnaissance a reference sketch of the ground should be prepared. Marking and fixing survey stations After selected the survey stations, they should be marked to enable them to be easily discovered during the progress of the survey. Running survey lines After having completed the preliminary work, the chaining may be started from the base line. The work in running a survey line is two fold.  to chain the line  to locate the adjacent details Basic Problems in chaining To erect a perpendicular to a chain line from a point on it Method-I AB is the chain line. It is required to erect a perpendicular to the chain line at point ‘C’ on it. Establish a point ‘E’ at a distance of 3m from ‘C’. Take 10m tape and put the zero (0) end of the tape at ‘E’ and the 10m end at ‘C’. The 5th and 6th meter marks of the tape are brought together to form a loop of 1m. The tape is now stretched tight by fastening the ends E and C. The point ‘D’ is thus established. Angle DCE will be 90º. 12 Method -II Select E and F equidistant from C (Fig (b)). Hold the zero end of the tape at E, and 10m end at F. Pick up 5m mark, stretch the tape tight and establish D. Join DC. To drop a perpendicular to chain line from a point outside it Method-I AB is the chain line. It is required to drop a perpendicular to a chain line AB from point ‘D’ out side it. Select any point ‘E’ on the chain line. With ‘D’ as center, and DE as radius, draw an arc to cut the chain line at F. Bisect EF at ‘C’ and then CD is perpendicular to chain line AB. Method –II Select any point E on the line (Fig(b)). Join ED and bisect it at F. With F as centre and EF or FD as radius, draw an arc to cut the chain line in C, CD will be perpendicular to the chain line. To run a parallel to chain line through a given point Method-1 AB is the chain line, C is the given point through which parallel line is to be drawn. Through ‘C’ drop a perpendicular CE to the chain line. Measure CE. Select any other point F on the line and erect a perpendicular FD. Make FD=EC. Join CD. Method-II Select any point F on the chain line (Fig(b)). Join CF and bisect at G. Select any other point E on the chain line. Join EG and prolong it to D such that EG = GD. Join C and D. Obstacle to chaining and ranging In chain surveying, sometimes the chainman is unable to measure the distance between two points directly, due to obstacles. Hence it has to be found out by indirect measurements. Basically there are three types of obstacles  Obstacle to ranging but not chaining  Obstacle to chaining but not ranging  Obstacle to both chaining and ranging Obstacle to ranging but not chaining In this type of obstacle, the ends are not intervisible and are quite common, except in a flat country. AB is the line in which A and B are not visible from intermediate point on it. Through A, draw a random line AB1 in any convenient direction but as nearly towards B as possible. The point B1 should be chosen in such a way that B1 is visible from B and BB1 is perpendicular to the random line. Measure BB1. Select C1 and D1 on the random line and erect perpendicular C1C and D1D on it. Join CD and prolong. 13 Obstacle to chaining but not ranging There are two types of obstacle in this case  When it is possible to chain round the obstacle. E.g., pond.  When it is not possible to chain round the obstacle. E.g., river When it is possible to chain round the obstacle Select two points A and B on either side (Fig(a)). Set out equal perpendiculars AC and BD. Measure CD; then CD=AB. Method-II Set out AC perpendicular to the chain line (Fig(b)). Measure AC and BC. The length AB is calculated from the relation. When it is not possible to chain round the obstacle Method-I Select point B on one side and A and C on the other side(Fig(a)). Erect AD and CE as perpendiculars to AB and range B, D and E in one line. Measure AC, AD and CE. If a line DF is drawn parallel to AB, cutting CE in F perpendicularly, then triangles ABD and FDE will be similar. Method-II Erect a perpendicular AC and bisect it at D (Fig(b)). Erect perpendicular CE at C and range E in line with BD. Measure CE. Then AB = CE Obstacle to both chaining and ranging Method-I Building is the typical example for this type of obstacle. Select two points A and B on one side of the obstacle. Erect perpendiculars AC and BD of equal length. Join CD and prolong it past the obstacle. Choose two points E and F on the line CD and erect perpendiculars EG and FH equal to that of AC or BD. Join GH and prolong it. Measure DE. BG = DE. Method-II Select a point A and erect a perpendicular AC of any convenient length(Fig(b)). Select another point B on the chain line such that AB=AC. Join B and C and prolong it to any convenient point D. At D, set a right angle DE such that DE=DB. Choose another point F on DE such that DE=DC. With F as centre and AB as radius, draw an arc. With E as centre, draw another arc of the same radius to cut the previous are in G. Join GE which will be in range with the chain line. Measure CF. Then AG=CF. 14 Compass surveying Introduction Chain surveying can be used when the area to be surveyed is comparatively small and is fairly flat. However, when large areas are involved methods of chain surveying alone are not sufficient and convenient. In such cases it becomes essential to use some sort of instruments which enables angles or direction of survey lines to be observed. In engineering practice, following are the instruments used for the measurements. Instruments for the direct measurement of direction  Surveyor’s compass  Prismatic compass Instruments for measurement of angles  Sextant  Theodolite Traverse survey Traversing is that type of survey in which number of connected survey lines form the frame work, and the direction and lengths of the survey lines are measured with the help of an angle measuring instrument and a tape respectively. When the line from a circuit which ends at the starting point it is known as a closed traverse, if the circuit ends from elsewhere, it is said to be open traverse. Bearing and angles The direction of a survey line can either be established  with relation to each other  with relation to any meridian The first one gives the angle between two lines, while second will give bearing of the line. Bearing Bearing of a line is its direction relative to a given meridian. Angle An angle is the difference in direction of two intersecting lines. Systems of angular measurement There are three systems of angular measurements Sexagesimal system 1Circumference=600degree 1degree=60’(minutes) 1minute=60”seconds Centesimal system 1Circumference=400”grade 1grade=100ccentigrade 1centigrade=100cc centi centigrade Hour system 1Circumference=24”(hour) 1hour=60minute 1 minute = 60 seconds Meridian A meridian is any direction. There are three meridians  True meridian 15   Magnetic meridian  Arbitrary meridian True meridian True meridian through a point is the line, in which a plane passing through that point and the north and south poles, intersects with the surface of the earth. It, thus passes through the true north and south. True bearing True bearing of a line is the horizontal angle, which it makes with the true meridian passing through one of the extremities of the line. Magnetic meridian Magnetic meridian through a point is the direction shown by a freely flatting and balanced magnetic needle free from all other attractive forces. The direction of magnetic meridian can be established with the help of a magnetic compass. Magnetic bearing The magnetic bearing of a line is the horizontal angle, which it makes with the magnetic meridian passing through one of the extremities of the line. Arbitrary meridian Arbitrary meridian is any convenient direction towards prominent mark or signal, such as church spire or top of a chimney. Arbitrary bearing Arbitrary bearing of a line is the horizontal angle which it makes with any arbitrary meridian passing through one of the extremities of the line. Whole circle bearing system and Reduced bearing system The common systems of notation of bearings are  The Whole Circle Bearing system (W.C.B) or Azimuthal system.  The Quadrantal bearing system or Reduced bearing system. The whole circle bearing system 16 The reduced bearing system Conversion of bearing from one system to other system From the figure 1, the conversion of whole circle bearing system (W.C.B) to reduced bearing system (R.B) Conversion of Reduced bearing system into whole circle bearing system Fore bearing, back bearing, conversion one bearing to other bearing Fore bearing and Back bearing The bearing of a line, whether expressed in WCB system or in Q.B system, differs according as the observation is made from one end of the line or from the other. If the bearing of line AB is measured from A towards B, it is known as forward bearing or Fore bearing. If the bearing of the line AB is measured B towards A is known as Backward bearing or back bearing. Since it is measured from backward direction. Considering W. C.B system, From fig 1 Fore bearing of the line AB is θ, back bearing of line AB = Φ, evidently of Φ= θ +180° and From fig 2 the back bearing of CD is Φ and fore bearing is θ. Hence Φ = θ – 180°. In general it can be stated that B.B = F.B +__ 180°.Using plus sign when F.B is less than 180°. and minus sign when F.B is greater than 180°. 17 Consider the Reduced Bearing system. From Fig 3 the fore bearing of the line AB is N θ E and the back bearing of the line is S θ W. Similarly from the fig 4, the fore bearing is of the line CD is S θ W and back bearing is called to N θ E.Thus it can be staled that to convert the fore bearing to back bearing it is only necessary to change the cardinal points by substituting N for S, and E for W and vice versa, the numerical value of the bearing remaining same. Calculation of angle from bearings From fig (a) the included angle α between the lines AB and AC is θ 2 - θ1 = Fore bearing of one line- fore bearing of the other line It is possible when the bearing being measured from common point A Fig (b) The angle α = (180° + θ1) – θ2 Back bearing of previous line – Fore bearing of next line Let us consider Reduced Bearing system from Fig (a) both the bearings have been measured to the same side of common meridian, the included angle α = θ 2 - θ1 , In Fig (b) both the bearing have been measured to the opposite sides of the common meridian = α = θ1+ θ2, From Fig (c)both the bearings have been measured to the same side of different meridians and the included angle α = 180°-(θ2 + θ1) In Fig (d) both the bearings have been measured to the opposite sides of the different meridians and angle α = 180°+ θ2 – θ1. Calculation of bearings from the angles In the case of traverse in which the included angles between successive lines have been measured the bearing of the lines can be calculated provided the bearing of line is also measured. Let α, β, γ, δ be the included angles measured clockwise from back stations and θ1 be the measured bearing of the line A.B. 18 The bearing of the next line BC = Bearing of the line AB + included angle at ‘B’ +- 180°= θ1 + α + 180° The bearing of the line CD = Bearing of the line BC + included angle at C + 180°= θ 2 + β +_ 180° The bearing of the line DE = Bearing of the line CD + included angle at D +_ 180° = θ 3 γ +_180° The bearing of the line EF= θ5 = Bearing of the line DE + included angle at E +_ 180° = θ4 + δ +_ 180° To get the bearing of the next line, add the measured clockwise angles to the bearing of the previous line. If the sum is more than 180°, deduct 180°, if the sum is less than 180° add 180° Theory of magnetic compass Magnetic compass gives directly the magnetic bearing of line. The bearing may either be measured in the W.C.B system or R.B system. The general principles of all magnetic compass depends upon the fact that, if a long, narrow strip of steel or iron is magnetized and is suitably suspended or pivoted about a point near its centre. So that it can oscillate freely about the vertical axis, it will tend to establish if self in the magnetic meridian at the place of observation. The most essential features of a magnetic compass are  Magnetic needle - to establish the magnetic meridian  A line of sight -To sight the other end of the line  A graduated circle - Either attached to the box or to needle to read the direction of the lines  A compass box - To house above parts  A tripod - used to support the box Following are the different types of compass  Surveyor’s compass  Prismatic compass  Transit or level compass The prismatic compass 19 Prismatic compass is the most convenient and portable form of magnetic compass which can either be used as a hand instrument or can be fitted on a tripod. Prismatic compass having a magnetic needle is attached to the circular ring or compass card made up of aluminium a non magnetic substance. When the needle is on the pivot it will orient itself in the magnetic meridian. The line of sight is defined by the object vane and the eye slit both attached to the compass box. The object vane consists of a vertical hair attached to a suitable frame while the eye slit consists of a vertical slit cut into the upper assembly of the prism unit, both being hinged to the box. When an object is sighted the sight vanes will rotate with respect to the NS end of ring through an angle which the line makes with magnetic meridian. A triangular prism is fitted below the eyes slit, having suitable arrangement for focusing to suit different eye slits. The prism has both horizontal and vertical faces convex, so that magnified image of the ring graduation is formed. The 00 or 3600 reading is therefore engraved on the south end of the ring, so that bearing of the magnetic meridian is read as ‘0’ with help of prism which is vertically above south end in this particular position. The readings increase in clockwise direction from 00 at south end to 900 at the west end, 1800 at north and 2700 at east end. If the instrument is not uses, the object vane can be folded on the glass lid which covers the top of the box. To sight the objects which are too high or too low to be sighted directly a hinged mirror capable of sliding over the object vane is provided and the objects sighted by reflection when bright objects are sighted dark glasses may be interposed into the line of sight. The main advantage of prismatic compass is that both sighting the object as well as reading circle can be done simultaneously without changing the position of the eye. Adjustment of prismatic compass There are two types of adjustments  Temporary adjustment  Permanent adjustment Temporary adjustment Temporary adjustment consists of  Centring  Levelling  Focussing the prism Centring: Centring is the process of keeping the instrument exactly over the station. The centring is invariably done by adjusting the tripod legs. A plumb-bob may be used to judge the centring and it is not available, it may be judged by dropping a pebble from centre of the bottom of the instrument. Levelling: If the instrument is a hand instrument it must be held in hand in such a way that the graduated disc is swinging freely and appears to be level as judged from the top edge of the case. Generally a tripod is provided with ball and socket arrangement with the help of which the top of the box can be leveled. Focussing the prism: The prism attachment is slided up or down for focusing till the readings are seen to be sharp and clear. Permanent adjustment: Permanent adjustment are those adjustment which are done only when fundamental relation between the parts are disturbed. 20 Problems 1. Determine the values of included angles in the closed compass traverse ABCD, conducted in clock wise direction, given the following fore bearings of their respective lines. Line Forebearing AB40º BC70º CD210º DA 280º Included angle = Back bearing of the previous line - Fore bearing of the next line A = B.B line AD – F.B of line AB= (280º -180º) - 40º = 60º B = B.B of line AB – F.B of line BC= (40º + 180º) – 70 º = 150º C = B.B of line BC – F.B of line CD (70º + 180º )-120º = 40º D = B.B of the line CD – F.B of line DA = (210º – 180) -280 + 360º = 110º Check : (2n -4) * 90º = (2 x 4 - 4) x 90º =360º A+B+C+D+E = 60º + 150º + 40º + 110º = 360º Problems 2. The following angles were observed in clockwise direction in an open traverse, ∟ABC = 124º 15”, ∟BCD = 156º 30’, ∟CDE = 102º, DEF = 95º, and ∟EFG = 215º 45’ Magnetic bearing of the line AB was 241º 30’ what would be the bearing of the line FG Bearing of the line BC = Bearing of the line AB+ ∟B + 180º = 241º 30’ +124º 15’ -180º =185º 45’ Bearing of the line CD = Bearing of the line BC + ∟C+180º = 185º 45’ +156º 30’-180º =162º 15’ Bearing of the line DE = Bearing of the line CD + ∟D+ 180º = 162º 15’ + 102º – 180º = 84º 15’ Bearing of the line EF = Bearing of the line D E + ∟E + 180º = 84º 15’ + 95º 15’ -180º =359º 30’ Bearing of the line FG = Bearing of the line E F + ∟F + 180º = 359º 30’+215º 45’- 360º - 180º = 35º 15’ Levelling Definition Levelling is the branch of surveying the object of which is  To find the elevation of given points with respect to a given or assumed datum.  To establish points at a given elevation or at different elevations with respect to given or assumed datum. Levelling deals with measurement in a vertical plane. Level Surface A level surface is defined as a curved surface at which each point is perpendicular to the direction of gravity at the point. The surface of still water is truly level surface. Level Line A level line is a line lying on a level surface. It is normal to the plumb line at all points. 21 Horizontal Plane Horizontal plane through a point is a plane tangential to the level surface at that point. It is perpendicular to the plumb line through the point. Horizontal Line It is a straight line tangential to the level line at a point. It is perpendicular to the plumb line. Vertical Line It is a line normal to the level line at a point. It is commonly considered to be the line defined by a plumb line. Datum Datum is any surface to which elevations are referred. The mean sea level offords a convenient datum world over and elevations are commonly given as so much above or below sea level. Elevation The elevation of a point on or near the surface of the earth is its vertical distance above or below an arbitrarily assumed level surface or datum. The difference in the elevation between two points is the vertical distance between the two level surfaces in which the two points lie. Vertical Angle Vertical angle is an angle between two intersecting lines is a vertical plane. Mean Sea Level Mean sea level is the average height of the sea for all stages of the tides. At any particular place it is derived by averaging the hourly tide heights over a long period of 19 years. Bench Mark Bench mark is a relatively permanent point of reference whole elevation with respect to some assumed datum is known. It is used either as a starting point for leveling or as a point upon which to close as a check. Methods of levelling There are three methods of levelling.  Barometric levelling  Trignometric levelling  Spirit levelling Barometric levelling Barometric levelling makes use of the phenomenon that the difference in elevation between two points is proportional to the difference in atmospheric pressure at these points. At a given point, the atmospheric pressure does not remain constant in the course of the day, even in the course of an hour. This method is therefore relatively in accurate. Trignometric levelling (Indirect levelling): Trignometric levelling is the process of levelling in which the elevations of points are computed from the vertical angles and horizontal distances measured in the field. Spirit levelling or Direct levelling It is the branch of levelling in which vertical distances with respect to a horizontal line may be used to determine the relative difference in elevation between two adjacent points. Levelling instruments: The instruments commonly used in direct levelling are  A level  A levelling staff 22 A level: The purpose of a level is to provide a horizontal line of sight. A level consists of the following four parts.  A telescope – To provide line of sight  A level tube – To make the line of sight horizontal  A levelling head – To bring the bubble in its centre of run  A tripod – To support the instrument There are four types of level  Dumpy level  Wye (Y) level  Reversible level  Tilting level Terms and abbreviations Station In levelling, a station is a point where the level rod is held and not where level is set up. It is the point whose elevation is to be ascertained or the point that is to be established at a given elevation. Height of Instrument (H.I) For any set up of level, the height of instrument is the elevation of plane of sight (line of sight) with respect to assumed datum. It does not mean that height of telescope above the ground where the level stands. Back Sight (B.S) Back sight is the sight taken on a rod held at a point of known elevation, to ascertain the amount by which the line of sight is above that point and thus to obtain the height of instrument. It is also known as plus sight as the back sight reading is always added to the level of the datum to get the height of the instrument. The object of back sighting is therefore, to ascertain the height of the plane of sight. Fore Sight (F.S) Foresight is a sight taken on a rod held at a point of unknown elevation, to ascertain the amount by which the point is below the line of sight and thus to obtain the elevation of the station. It is known as minus sight as the foresight reading is always subtracted from the height of instrument to get the elevation of the point. The object of fore sighting is therefore to ascertain the elevation of the point. Turning Point (T.P) Change Point: Turning point is point on which both minus sight and plus sight are taken on a line of direct levels. The minus sight is taken on the point in one set of instrument to ascertain the elevation of the point, while the plus sight is taken on the same point in other set of the instrument to establish the new height of instrument. Intermediate Station (I.S) Intermediate station is a point, intermediate between two turning points on which only one sight is taken to determine the elevation of the station. Types of spirit levelling Differential levelling It is the method of direct leveling, the object of which is solely to determine the difference in elevations of two points regardless of the horizontal positions of the points with respect to each other. When the points are apart, it may be necessary to set up the instrument several times. This type of levelling is also known as fly- levelling. Profile levelling It is the method of direct levelling, the object of which is to determine the elevations of points at measured intervals along a given line in order to obtain a profile of the surface along that line. 23 Cross–sectioning Cross-sectioning or cross–levelling is the process of taking levels on each side of a main line at right angles to that line in order to determine a vertical cross – section of the surface of the ground. Reciprocal levellingIt is the method of levelling in which the difference in elevation between two points is accurately determined by two sets of reciprocal observation when it is not possible to set up the level between two points. Precise levelling It is the levelling in which the degree of precision required is too great, is attained by ordinary methods and in which, therefore special equipment or precautions or both are necessary to eliminate as far as possible all sources of error. Find out the elevation of points using different methods Steps in levelling There are two steps in levelling  To find by how much amount the line of sight is above the bench mark.  To ascertain by how much amount the next point is below or above the line of sight. A level is set up approximately mid way between the benchmark and the point of elevation of which is to be ascertained by direct levelling. A back sight is taken on a rod held at the bench mark. H.I. = Elevation of the Benchmark + Back sight Turning the telescope to bring into view the rod held on the point “B” a foresight is taken. ELEVATION = H.I – Fore sight Eg: Bench mark = 210.852 m. Back sight = 2.324 m Fore sight = 1.836 m H.I = Elevation of B.M + B.S = 210.852+2.324=213.176m. Elevation of the point “B” = H.I – Fore sight = 213.176-1.836=211.340 m. It is to be noted that if a back sight is taken on a bench mark located on the roof of a tunnel or on the ceiling of a room with the instrument at a lower elevation the back sight must be subtracted from the elevation to get the height of the instrument. Similarly if the foresight is taken on a point higher than the instrument, the fore sight must be added to the height of the instrument to get the elevation of the point. Differential levelling Levelling is to determine the elevation of points at some distance apart is called “differential levelling”. When two points are at such a distance from each other that they cannot both be within range of level at the same time the difference in elevation is not found by single setting but the distance between the points is 24 divided in two stages by turning points on which the staff is held and the difference of elevation of each of succeeding pair of such turning points is found by separate setting up of the level. A and B are the two points. The distance AB has been divided into three parts by choosing two additional points on staff readings has been taken. Points 1 and 2 are turning points. Reduced level of point A = 240.000m. The height of the first setting of the instrument is therefore 240.000+2.024 = 242.024m. If the following Fore sight is 1.420. The R.L of TP1=242.024-1.420=240.604 m. The back sight for the second set up of instrument is 1.986. The H.I for second set up is = 240.604+1.986=242.590 m. By similar process of calculations R.L. of TP2 = 240.490 and R.L of B = 241.202. Booking and Reducing Levels There are two methods of booking and reducing the elevation of points from the observed staff readings  Collimation or Height of instrument method  Rise and fall method. Height of Instrument method  In this method, the height of the instrument (H.I) is calculated for each setting of the instrument by adding back sight (plus sight) to the elevation of the B.M.  The elevation of (Reduced Level) the turning point is then calculated by subtracting fore sight from H.I the.  For the next setting of the instrument the H.I. is obtained by adding the B.S taken on TP1 to its R.L.  The process continues till the R.L of the last point (fore sight) is obtained by subtracting the staff reading from height of the last setting of the instrument.  If there are some intermediate points the R.L of those points is calculated by subtracting the intermediate sight (Minus sight) from the height of the instrument for that setting. Check The difference between the sum of back sight and sum of the fore sights should be equal to the difference between the last and first R.L. ∑B.S – ∑F.S = Last RL – First RL 1. The following staff readings were observed successively with a level, the instrument having been moved after, second, fifth and seventh readings. 0.865, 2.105, 1.025, 1.580, 1.865, 2.230, 2.835, 2.355, 1.760 Enter the above readings in a page of a level book and calculate the R.L of points, if the reading was taken with a staff held on a B.M. of 560.500 25 ∑ B.S = 6.475 ∑ F.S = 8.565 Check = ∑ BS - ∑ FS = First R.L - Last R.L = 6.475-8.565 = 560.500-558.41 2.09= 2.09 Rise and Fall method In this method the height of instrument is not at all calculated but the difference of level between consecutive points is found out by comparing the staff reading on the two points for the same setting of the instrument. The difference between their staff readings a rise or fall according as the staff reading at the point is smaller or greater than that at the proceeding point. The figures for rise and fall worked out thus for all the points give the vertical distance of each point above or below the preceding one, and if the level of any one point is known the level of the next will be obtained by adding its rise or subtracting its fall as the case may be. Check The difference between the sum of back sight and sum of the fore sights should be equal to the difference between the sum of rise and sum of fall and should also be equal to the difference between the first R.L. and Last R.L. ∑ BS - ∑FS = ∑Rise - ∑Fall = First R.L.- Last R.L. 2. The following staff readings were observed successively with a level, the instrument having been moved after second, fifth and seventh readings. 0.865, 2.105, 1.025, 1.580, 1.865, 2.230, 2.835, 2.355, 1.760 Enter the above readings in a page of a level book and calculate the R.L. of points if the first reading was taken with a staff held on a B.M. of 560.500 (Use rise and fall method) Check = ∑BS - ∑FS = ∑rise - ∑Fall = Frits RL – Last RL = 6.475 – 8.565= 0.595-2.685 = 558.41-560.500 = 2.09 = 2.09 = 2.09 26 Problems 3. The following staff readings were observed successively with a level, the instrument having been moved after third, sixth and eighth readings. 2.228, 1.606, 0.988, 2.090, 2.864, 1.262, 0.602, 1.982, 1.044, 2.684 m. Enter the above readings in a page of a level book and calculate the R.L. of points, if the first reading was taken with a staff held on a bench mark of 432.384 m. Check = ∑BS-∑FS = ∑rise = ∑fall = first RL – last RL =5.964=6.916=2.842=3.794=432.384-431.432 =0.952 0.952 0.952 4. The following consecutive readings were taken with a level and 5 m. levelling staff on continuously sloping ground at common intervals of 20 m. 0.385, 1.030, 1.925, 2.825, 3.730, 4.685, 0.625, 2.005, 3.110, 4.485 The reduced level of the first point was 208.125. Rule out a page of level field book and enter the above readings. Calculate the reduced levels of the points by rise and fall method. Check = ∑B.S. = ∑F.S. = First R.L = Last R.L 1.01 = 9.17 = 208.125 = 199.965 = 8.16 = 8.16 27 Levelling Staff Levelling staff in a straight rectangular rod having graduations the foot of the staff representing zero reading. The purpose of a level is to establish a horizontal line of sight. The purpose of levelling staff is to determine the amount by which the station (foot of the staff ) is above or below the line of sight. Levelling staffs may be classified into two classes.  Self reading staff  Target staff Self reading staff-: is the one which can be read directly by the instrument man through the telescope. Target staff: on the other hand, contains a moving target against which the reading is taken by a staff man. There are usually three forms of self reading staff  Solid staff  Folding staff  Telescopic Solid staff The staff is generally made up of well seasoned wood having a length of 10 feet or 3m. In most common forms the smallest division is of 0.01feet or 5 mm. However some staves may have fine graduation up to 2 mm. These staves gives the reading is English and metric units. Folding staff Folding staff usually 10ft long having a hinge at the middle of its length. When the staff is not in use the rod can be folded about the hinge so that it becomes convenient to carry it from one place to the other. Since the self-reading staff is always seen through the telescope all readings appear to be inverted. The readings are therefore taken from above downwards. Telescopic staff It is arranged in three telescopic lengths. When fully extended it is usually of 14ft length. The 14 ft staff has solid top length 4’6” sliding in to the central box of 4’ 6” length. The central box in turn slides into the lower box of 5’ length. In the 5 m staff the three corresponding length are usually 1.5, 1.5 and 2 m. The levelling staves graduated in English unit generally have whole number of feet marked in red to the left side of the staff. The odd lengths of the feet are marked in black to the right hand side. The top of these black graduations indicates the odd length, while the bottom shows the even length. The hundredth feet are indicated by alternate white and black spaces, the top of a black space indicating odd hundredth feet and top of a white space indicating even hundredth. Some times when the staff is near the instrument the red mark of whole foot may not appear in the filed of view. In that case the staff is raised slowly until the red figure appears in the field of view, the red figure thus indicating the whole feet. Folding level staff in metric units Folding level staff is in 4 m length. The staff comprises 2 m thoroughly seasoned wooden pieces with the joint assembly. Each piece of the staff is made of one longitudinal strip without any joint. The width and thickness of staff is kept 75 mm and 18mm respectively. The folding joint of the staff is made of the detachable type with a locking device at the back. The staff is joined together is such a way that  The staff may be folded to 2 m length.  The two pieces maybe detached from on another when required facilitating easy handling and manipulating on with one piece.  When the two portions are locked together the two pieces become rigid and straight. Each meter is subdivided into 200 divisions the thickness of graduations being 5mm. 28 Adjustment of a level  Permanent adjustment  Temporary adjustment Permanent adjustment made only when the fundamental relations between some parts or lines are disturbed. Temporary adjustment or station adjustment, which are made at every instrument setting and preparatory to taking observations with instrument.Temporary adjustment consists of  Setting up the level  Levelling up.  Elimination of parallax Setting up the level  The operation of setting up includes  Fixing the instrument on the stand Levelling the instrument approximately by leg adjustment To fix the level to the tripod, the clamp is released; instrument is held in the right hand and is fixed on the tripod by turning round the lower part with the left hand. The tripod legs are so adjusted that the instrument is at the convenient height and the tribrach is approximately horizontal. Levelling up After having levelled the instrument approximately accurate levelling is done with the help of foot screws and with reference to the plate levels. The purpose of levelling is to make the vertical axis truly vertical. The manner of levelling the instrument by the plate levels depends upon whether there are three or four leveling screws. Three screw head  Loose the clam. Turn the instrument until the longitudinal axis of the plate level is roughly parallel to a line joining any two of the leveling screws (A and B)  Hold these two leveling screws between the thumb and first finger of each hand and turn them uniformly so that the thumbs move either towards each other or away from each other, until the bubble is central. It should be noted that the bubble will move in the direction of movement of the left thumb  Turn the upper plate through 900 until the axis on the level passes over the position of the third levelling screw, ‘C’  Turn this levelling screw until the bubble is central  Return the upper plate through 900 to its original position and repeat (step-2) till the bubble is central.  Turn back again through 900 and repeat step (4) 29   Repeat steps (2) and (4) till the bubble is central in both the positions.  Rotate the instrument through 1800. The bubble should remain in the centre if its run provided it is in correct adjustment. Elimination of parallax  Parallax is a condition arising when the image formed by the objective is not in the plane of the cross-hairs. Unless parallax in eliminated, accurate sighting is impossible. Parallax can be eliminated is two steps. i) By focusing the eye-piece for distinct vision of the cross – hairs. ii) By focusing the objective to bring the image of the object in the plane of cross – hairs. Focusing the eye piece for distinct vision of cross-hairs To focus the eye piece for distinct vision of the cross-hairs, point the telescope to words sky (or hold a piece of white paper in front of the objective) and move eye-piece in or out till the cross hairs are seen sharp and distinct. Focusing the eye piece to bring the image of the object The telescope is now directed towards the staff and the focusing screw is turned till the image appears clear and sharp. The image so formed is in the plane of cross hairs. Plane table surveying Definition Plane table surveying is the graphical method of survey in which the field observations and plotting proceed simultaneously. It is a making of manuscript map in the field, while the ground can be seen by the topographer and without intermediate steps of recording and transcribing field notes. Instruments used for plane table surveying  Plane table with levelling head having arrangements for i) Levelling ii) Rotation about vertical axis iii) Clamping in any required position  Alidade for sighting  Plumbing fork and plumb bob  Spirit level  Compass  Drawing paper with a rainproof cover The plane table The plane table consists of a small drawing board mounted on a light tripod in such a way that the boards can be rotated about the vertical axis and can be clamped in any position.There are three types of plane table,  Traverse table  Johnson table  Coast survey table Alidade A plane table alidade is a straight edge with some form of sighting devices. It generally consists of a metal or wooden rule with two vanes at the ends. The two vanes or sights are hinged to fold down on the rule when the alidade is not in use. One of the vane is provided with a narrow slit, while the other is open and carries a hair or thin wire. Both the slits thus provide a definite line of sight which can be made to pass through the object to be sighted. The alidade can be rotated about the point representing the instrument station on the sheet so that the line of sight passes through the object to be sighted. A line is then drawn 30 against the working edge of the alidade. The alidade is not very much suitable on hill area since the inclination of the line of sight is limited. There are two types of alidade.  Plane alidade  Telescopic alidade. Plumbing Fork The plumbing fork used in large scale work, is meant for centring the table over the point or station occupied by the plane table when the plotted position of that point is already known on the sheet. Also in the beginning of the work, it is meant for transferring the ground point on to the sheet so that plotted point and the ground station are in the same vertical line. The fork consists of a hair pin- shaped light metal frame having arms of equal length in which a plumb bob is suspended from the end of lower arm. The fitting can be placed with the upper arm lying on the top of the table and the lower arm below it, the table being centered when the plumb bob hangs freely over the ground mark and the pointed end of the upper arm coincides with the equivalent point on the plan. Spirit level A small spirit level may be used for ascertaining if the table is properly level. The level may be either of the tabular variety or of the circular type essentially with a flat base so that it can be laid on the table and is truly level when the bubble is central. The table is levelled by placing the level on the board in two positions at right angle and getting the bubble central in both positions. Compass The compass is used for orienting the plane table to magnetic north. The compass used with a plane table is a trough compass in which the longer sides of the trough are parallel and flat so that either side can be used as a ruler or laid down to coincide with a straight line drawn on the paper. Drawing paper The drawing paper used for plane tabling must be of superior quality so that it may have minimum effect of changes in the humidity of the atmosphere. The changes in the humidity of the atmosphere produce expansion and contraction in different directions and thus alter the scale and distort the map. To overcome this difficulty, sometimes two sheets are mounted with their grains at right angles and with a sheet of muslin between them. Single sheet must be seasoned previous of the use by exposing it alternatively to a damp and a dry atmosphere. Working operations Three operations in plane table surveying  Fixing : Fixing the table to the tripod  Setting : i) Levelling the table ii) Centring iii) Orientation  Sighting the points Fixing Fix the plane table to the tripod properly Setting Levelling for small scale work, levelling is done by estimation. For work of accuracy an ordinary spirit level may be used. The table is levelled by placing the level on the board in two positions at right angles and getting the bubble central in both directions. 31 Centring The table should be so placed over the station on the ground that the point plotted on the sheet corresponding to the station occupied should be exactly over the station on the ground. The operation is known as centring the plane table. Orientation Orientation is the process of putting the plane-table into some fixed direction so that a line representing a certain direction on the plane is parallel to the direction on the ground. This is the essential condition to be fulfilled when more than one instrument station is to be used. If orientation is not done, the table will not be parallel to itself at different positions resulting in an overall distortion of the map. The process of centring and orientation dependent on each other. For exact work the centirng and orientation is repeat until the work is accurate. There are two main methods of orientation of the plane table  Orientation by means of trough compass.  Orientation by means of back sighting. Orientation Orientation by means of trough compass The plane table can be oriented by compass under the following conditions.  When speed is more important than accuracy.  When there is no second point available for orientation  For approximate orientation prior to final adjustment. For orientation, the compass is so placed on the plane table that the needle floats centrally and a fine pencil line in ruled against the long side of the box. At any other station where the table is to be oriented, the compass is placed against thin line and the table is oriented by turning it until the needle floats centrally. The table is then clamped is position. Orientation by back sighting To orient the table at the next station say B represented on the paper by a point “b” plotted by means of a line “ab” drawn from a previous station A, the alidade is kept on the line “ba” and the table is turned about its vertical axis in such a way that the line of sight passes through the ground station “A”. When this is achieved the plotted line “ab” will be co-insiding with the ground line AB and the table will be oriented. The table is then clamped line position. Sighting the points When once the table has been set, i.e., when levelling centring and orientation has been done, the points to be located are sighted through the alidade. The alidade is kept pivoted about the plotted location of the instrument station and is turned so that the line of sight passes or bisects the signal at the point to be plotted. A ray is then drawn from the instrument station along the edge of the alidade. Similarly the rays to other points to be sighted are drawn. Methods of plane tabling Methods of plane tabling can be divided into four distinct heads.  Radiation  Intersection  Traversing  Resection The first two methods are generally employed for locating the details, while the other two methods are used for locating the plane table station. 32 Radiation In this method, a ray is drawn from the instrument station towards the point, the distance is measured between the instrument station and that point, and the point is located by plotting to some scale the distance so measured. This method is more suitable when the distances are small and one single instrument station can control the points to be detailed. The following steps are necessary to locate the points from an instrument station T.  Set the table at T, level it and transfer the point on to the sheet by means of plumbing fork, thus getting “t” representing T. Clamp the table.  Keep the alidade touching “t” and sight to A, draw the ray along the fiducial edge of the alidade. Similarly sight different points B, C and D, E etc., and draw the corresponding rays (A pin may be inserted at “t” and the alidade may be kept touching the pin while sighting the points)  Measure TA, TB, TC, TD, TE etc., in the field and plot their distance to some scale along the corresponding rays, thus getting a, b, c, d, e etc., Join these if needed. Intersection Intersection is resorted to when the distance between the point and the instrument station is either too large or cannot be measured accurately due to some field conditions. The location of an object is determined by sighting the object from two plane table stations and drawing the rays. The intersection of these rays will give the position of the object. It is therefore very essential to have at least two instrument stations to locate any point. The distance between the two instrument stations is measured and plotted on the sheet to some scale. The line joining the two instrument stations is known as the “base line”. The following is the procedure to locate the points by the method of intersection.  Set the table at “A” level it and transfer the point “A” on to the sheet by way of plumbing fork. Clamp the table.  With the help of the trough compass mark the north direction on the sheet.  Pivoting the alidade about “a” sight it to “B”. Measure AB and plot it along ray to get “b”. The base line “ab” is thus drawn.  Pivoting the alidade at “a” sight the details C,D,E etc., and draw corresponding rays.  Shift the table at “B” and set it there. Orient the table roughly by compass and finally by back sighting “A”. 33   Pivoting the alidade about “b” sight the details C, D, E etc., and draw the corresponding rays along the edge of the alidade to interest with the previously drawn rays in C, D, E etc., The position of the points are thus mapped by way of intersection. Contour surveying Definition Contour is an imaginary line on the ground joining the points of equal elevation. It is a line in which the surface of ground is intersected by a level surface. “A contour line is a line on a map representing a contour”. Fig shows a pond with water at an elevation of 100 m. If the water is lowered by one meter another water mark representing 99.00 m elevation will be obtained. These water marks may be surveyed and represented on the map in the form of contours. A topographic map represents a clear picture of the surface of the ground. If a map is to a big scale it shows where the ground is nearly level where it is sloping, where the slopes are steep and where they are gradual. If a map is to a small scale, it shows the flat country. Contour interval The vertical distance between any two consecutive contours is called the contour interval. The contour interval is kept constant for a contour plan; otherwise the general appearance of the map will be misleading. The horizontal distance between two points on two consecutive contours is known as the horizontal equivalent and depends upon the steepness of the ground. The contour intervals depend upon the following considerations.  Nature of ground  Scale of the map  The purpose and extent of the survey  Time and expense of field and office work Nature of ground The contour interval depends upon whether the country in flat or highly undulated. For very flat ground, a small interval is necessary. If the ground is more broken, greater interval should be adopted, otherwise the contours will become too close to each other. Scale of the map The contour interval should be inversely proportional to the scale. If the scale is small, the contour interval should be large. If the scale is large the contour intervals should be small. The purpose and extent of the survey The contour interval largely depends upon the purpose and the extent of the survey. For example, if the survey is intended for detailed design work or for accurate earth work calculations small contour interval is to be used. In this case extent of survey will be generally small. In the case of location survey, for lines of communication for reservoir and drainage areas, where the extent of survey is large, a large contour interval is to be used. 34 Time and expense of field and office work If the time available is less, greater contour interval should be used. If the contour interval is small greater time will be taken in the field survey in reductions and in plotting the map. Characteristics of contour  Two contour lines of different elevation cannot cross each other. If they did, the point of intersection would have two different elevations which is absurd.  However contour lines of different elevations can intersect only in the case of overhanging cliff or a cave  Contour lines of different elevations can unite to form one line only in the case of a vertical cliff.  Contour lines close together in indicate steep slope. (a) They indicate a gentle slope if they are far a part. (b) If they are equally spaced uniform slope is indicated.  (c) A series of straight, parallel and equally spaced contours represent a plane surface. A contour passing through any point is perpendicular to the line of steepest slope at that point. The perpendicular distance between contour lines is the shortest distance.  A closed contour line with one or more higher inside it represent a hill. Similarly a closed contour line with one or more lower ones inside it indicates depression without an outlet. Two contour lines having the same elevation cannot unite and continue as one line. Similarly a single contour cannot split into two lines.  A contour line must close upon itself, though not necessarily within the limits of the map.  Contour lines cross a water shed or ridge line at right angles. They form curves of U-shape round it with the concave side of the curve towards the higher ground (a). Contour lines cross a valley line at right angles. They form sharp curves of v-shape across it with convex side of the curve to words the higher ground. If there is a stream the contour on either side, turning upstream, may disappear in coincidence with the edge of the stream and cross underneath the water surface. Methods of locating contours There are two methods for locating contours 35   The direct method  The indirect method In direct method, the contour to be plotted is actually traced on the ground. Only those points are surveyed which happed to be plotted. This method is slow and tedious. It is used for small areas and where great accuracy is required. In the indirect method some suitable guide points are selected and surveyed, the guide point need not necessarily be on the contours. These guide points having been plotted serve as a basis for the interpolation of contour. Direct method The direct method is divided into two forms.  Vertical control : Location points  Horizontal control: Survey of those points. Vertical control The points on the contours are traced with the help of a level and staff. The level is set at a point to command the area as much as possible and is levelled. The staff is kept on the B.M and the height of the instrument is determined. If the B.M is not nearby fly levelling may be performed to establish a temporary bench mark (T.B.M.) in that area. Having known the H.I the staff reading is calculated so that the bottom of the staff is at an elevation equal to the valve of the contour. E.g.: H.I =101.80 meter, the reading to get a point on the contour of 100.00 m elevation will be 1.80 m. Taking one contour at a time, the staff man is directed to keep the staff on the points on contour so that reading of 1.80 m is obtained every time. Horizontal control After having located the points on various contours they are to be surveyed with a suitable control system. The system to be adopted depends mainly on the type and extent of areas. For small areas chain surveying may be used and the points may be located by off sets from the survey lines. For larger work the theodolite compass or plane table may be used. Indirect method In this method, some guide points are selected along a system of straight lines and their elevations are found. The points are then plotted and contours are then drawn by interpolation. There are three indirect methods in locating contours.  By squares  By cross-sections  By Tachometric methods By squares This method is used when the area to be surveyed is small and the ground is not very much undulating. The area to be surveyed is divided into number of squares. The size of the squares may vary from 5 to 20 m depending upon the nature of the contour and contour interval. The elevations of the corners of the square are then determined by means of a level and a staff. The contour lines may be drawn by interpolation. 36 Interpolation of contour Interpolation of the contours is the process of spacing the contours proportionately between the plotted ground points established by indirect methods. The methods of interpolation are based on the assumption that the slope of ground between the two points is uniform. Following are the three methods of interpolation.  By estimation  By arithmetic calculation  By graphical method By estimation This method is extremely rough and is used for small scale work only. The position of contour points between the guide points are located by estimation. By arithmetic calculation This method so accurate and is time consuming. The positions of contour points between the guide points are located by arithmetic calculation e.g. A, B, C and D be the guide points plotted on the map. Elevations at each point are 607.4, 617.3, 612.5 and 604.3 respectively. Let AB=BD, CD=CA= one inch on plan. The vertical difference in elevation between A and B is (617.3-607.4) = 9.9 feet. Hence the distance of the contour points from A will be calculated as follows i.e., 1/x * y*z Where, x= Difference in contour elevation between two points y= The distance between two points z= The distance between the starting point to contour line Distance of 610 feet contour point says A1 is calculated by interpolation using the formula, The difference in contour elevation between two points is (617.3-607.4) = 9.9 feet. The distance between the two points = 2.0m The distance between the starting point to contour line is 610- 607.4 = 2.6 feet Distance from point ‘A’ is = (1/9.9) x 2.6 x 2 = 0.52 m These contour points may be located on AB the contour points for any lines can be calculated. Uses of contour Following are the some of the uses of contour map,  Drawing of section  Determination of inter-visibility between two points  Tracing of contour gradients and location of route  Measurement of drainage area  Calculation of reservoir capacity 37  Unit -2 FISH FARM Definition Fish farm is a set of scientifically planned, designed and constructed ponds for various fish cultural activities like breeding, hatching, rearing, nursing and stocking of fishes. Objectives of fish farms  For breeding of fishes  To maintain brooders  To conduct the research to improve fish variety and their culture  To make facilities to store required quantity of fish seeds  To demonstrate the fish cultural activities to fish culturists The success of fish culture is mainly depending upon proper planning and construction of pond. The design of the fish ponds are very much influenced by the type of pond and system of culture to be practised in the pond. Types of farms Classification of fish farm Fish farms can be classified into various types based on various factors. a) Based on the characteristics of the farm environment  Warm water  Cold water b) Water salinity  Fresh water  Brackish water  Marine water Fresh water is defined when the salt content in it is less than 0.5 ppt, brackish water is defined when the salt content in it is in the range of 0.50 – 30 ppt, marine water is defined when the salt content in it is more than 30-35 ppt. c) Water replacement  Running  Stagnant d) Physiographical zone  Inland fish farm  Coastal fish farm  Marine fish farm e) Kind of materials used for enclosure  Plastic tanks  Cement concrete  Earthen ponds  FRP ponds f) Based on water source  Rain fed farm 38   Tide fed farm  Sewage water  Seepage water  Ground water  Spring water  Municipal/corporation water  River/canal /dam g) Based on culture  Extensive  Semi – intensive  Intensive  Super intensive  Traditional culture Selection of site for Aqua farm Site selection criteria Selection of a suitable site is the first and fore most step in the design, planning, construction, operation and maintenance of fish farms. A mistake is made during the phase of site selection may result in higher cost of construction, culture operation and maintenance and may creates environmental problems also. Selection of a suitable site strongly influences the ultimate success of the aquaculture enterprise. The process of site selection is not only to determine the suitability of site, it is also valuable in determining the modifications required with regard to make farming possible at a given site. For high production and efficient management knowledge of local area and experience coupled with scientific and engineering expertise are also required. Although no site will poses all desired characteristics. Yet one has to select a site so as to obtain maximum production at minimum cost of construction and management. “A suitable site is one that provides optimum conditions for the growth of species cultured at the targeted production level given an effective pond design and support facilities”. Site selection criteria mainly depending upon the following points  The species to be cultured  The targeted production level  Culture technology  Investment Stages of site selection Selection of suitable site for an aqua farm can be done in two stages.  Reconnaissance survey (Pre-investment survey)  Detailed survey Reconnaissance survey/ Pre-investment survey While conducting reconnaissance or pre-investment survey, the following important points to be considered.  Accessibility  Physical features of the land  Soil characteristics  Water source 39 Accessibility The proposed site should be as far as possible near to the good, approachable road and market. During construction of farm it is easy to transport the construction materials to the site. It reduces the cost of construction and time. During culture it is easy to transport feed and fertilizer to the farm and also fish and its products to the market conveniently. It reduces the cost of production, which helps to increase the profit. Physical features of the land Physical features of the land are classified into two types. i) Natural feature ii) Artificial features. Natural features such as river, mountain, spring, forest, lakes, streams, rocks etc,. The artificial features are ponds, buildings, electrical pole, roads, rails etc., It is necessary to note down all these things and prepare a rough sketch. Soil characteristics Soil is the basic material for fish farms. Because in general the bottom of the ponds is soil, dykes are constructed by using soil to retain the water, reservoir may also built by soil to store the water and water is supply to the ponds through canals, and drainage canals to discharge unwanted, polluted water is also make up of soil. So it is very important to study some of the important properties such as fertility, texture, permeability and water holding capacity of soil. Water Source The proposed site should be as far as possible very near to the source of water. The source should be perennial. There should not be any shortage of water, particularly during summer to replenish the ponds. The water should be clean and well oxygenated. Detailed survey Important points to be considered during detailed survey. Main factors  Site condition  Topography  Soil characteristics  Water properties and supply Other factors  Environmental condition  Socio-economic condition  Pollution  Availability of materials  Technical assistance Main factors – Site condition The proposed site for pond construction should be essentially having sufficient area to accommodate all the desired number of ponds. The site is considered to be ideal for pond construction if the land surface is flat or has uniform gentle slope in one direction. It gives facilities for quicker filling and emptying of the pond water. From the economic point of view a pond should be located where the largest storage volume can be obtained with the least amount of earth fill. This condition generally will occur at a site where the valley is narrow, side slopes are relatively steep and slope of the valley floor will permit a large deep basin. 40 Avoid a site in a valley, relatively open and wide at down stream end. If so it is necessary to build long dam. It increases the cost of construction. Site should be good soil, which will hold the water well. The site should have the facility to adequate drainage arrangement. It is necessary to avoid forest area or the site with full of large number of trees. Other wise which may difficulty to remove and it leads to increase the cost of construction and also create problems during operation and maintenance. Dampy areas are also not suitable for pond construction because pond drainage is always difficult and much expensive. The optimum slope is between 0.5 to 1%. The ground slope should not be more than 2% as it needs construction of high and costly embankments. Shape of the site should be regular, irregular and oblong shape results in difficulty for planning, designing, construction, operation and maintenance of ponds. Sites with excessive undulating topography should be avoided as a lot of excavation and embankment would be needed during the construction, which will increase the cost of construction. The area should be sufficiently extensive to allow future expansion. Topography Topography is the science of measuring the earth and its features and making of maps, charts and plans to show them. A topographical map not only shows the location of the features but also shows the slope of the ground. Weather the ground is flat, having uniform gentle slope, steep slope or undulated aground etc., and how much steep in between these locations. Topographic map helpful to find out  Size and shape of the land  Slope of the land  Its elevation in relation to the water source  Distance between source of water and location of site  The best way of water supply to the ponds  The easiest way of draining the ponds Soil characteristics Soil characteristics play a vital role for selection of suitable site. The site should contain soft bottom soil or mixed soil comprising of clay, silt and sand in proper proportion to ensure good water holding capacity as well as production of natural food organisms on which aquatic organisms could feed and grow. One of the most important characteristics is the water holding capacity. Gravel and sand are non-cohesive soils. Their cohesiveness almost nil under dry condition and have no plasticity. In sand and gravel bed water percolates easily. Hence sand and gravel bed are not suitable for pond construction. Clay, silt and fine grained soils are cohesive soils. Cohesive property imparts structural stability in pond dykes, bottom etc., clay is very absorbent under wet condition, it swells to double its volume. So only clay, silt are also not that much suitable for pond construction. A combination of cohesive and non-cohesive soils such as sand, silt and clay in proper proportion are suitable for pond construction. According to textural classification of soil, clay loam, sandy clay loam, silty 41 clay loam and sandy clay soils are suitable for pond construction. Too much organic matter in the soil is harmful. Land with a layer of organic matter greater than 0.6 m deep is unsuitable for ponds. Because organic stratum will cause excessive seepage losses, when it decays. Highly organic soils are not suitable for dyke construction. Organic soils also cause rapid oxygen depletion in the pond water. Water properties and supply Once the source of water has been selected the next necessary step is to supply the water from the source to the fish ponds. Open channels, conduits and pipe lines are the three common means of conveyance of water. As far as possible the water supply should be made by gravity flow. As far as possible every pond should have its own inlet and outlet structure. So that it is easy to supply the water to the ponds when it required and discharges the unwanted or polluted water easily and conveniently. It is better to avoid direct entry of water from surface rivers, streams and springs to the fish ponds during rainy seasons. It may consist of clay and silt etc,. It may deposit on the pond bed which may increase the load on pond bed and sides of the dyke and also reduce the volume of the pond. Similarly it is better to avoid water from the agriculture lands. It may consist of chemicals, antibiotics and pesticides. It may affect aquatic animal’s life. Water quality management is ongoing and never ending process. It is very essential to study some of the important properties such as pH, temperature, dissolved oxygen, salinity, turbidity, alkalinity; acidity etc,. The management of water quality is the most important factor in the productive fish farming. An analysis of physical, chemical and biological properties of the proposed source of water must be conducted. A good quality is nothing but a web of physical, chemical and biological factors, which constitute the water environment and influence the production of fishes and shrimps. Other factors – Environmental condition (Meteorological) factors Meteorological parameters such as rain fall, its quantity, duration, intensity and type, evaporation, temperature, humidity, atmospheric pressure, winds, its speed, direction have a great role in the growth of fish food organisms as well as in the design of the aqua farms. A thorough hydrological survey should be conducted for the water source and area surrounded around the site Socio-economic condition Information regarding socio-economic conditions of the locality is important for managing farm efficiently. Details regarding seasonal availability of laboures, professionals like competent biologists, skilled labours, local customs traditions should be gathered. Man power planning mainly depends upon local wages and the availability of skilled labours. Pollution It is better to select the site away from the pollutant area. Industrial effluents, sewage out falls, insecticide affected agriculture land affect the air and water, and results in reduce the growth rate of aquatic animals. Available materials It is better to plan and design the farms for construction of the ponds and other facilities using locally available materials. It reduces the cost of construction and project time. Technical guidance Technical guidance from the fisheries department or private consultants help the farmers to solve common problems like excessive seepage, soil erosion, aquatic animal’s diseases etc,. A good technical guidance can even motivate the other farmers to take up fish culture. Farmers will come to know the latest researches, findings, development etc., if proper technical guidance is given. 42 Ponds Definition Ponds are the water bodies created by constructing a dam across a source of water or excavating a pit. Classification of ponds based on construction  Excavated ponds: These are the ponds created by excavating a pit across the source of water.  Embankment ponds: These are the ponds created by constructing a dyke or embankment across the water source Classification of ponds based on source of water Sunken ponds These ponds are not having inlet and outlet structures. These ponds are rain fed ponds. Barrage ponds These ponds are usually filled by rainfall or by spring water. A spring sends the water flowing through a small valley or down a slope into a low place or a spring bubbles form the ground into a natural depression. The pond is formed by collecting water at the base of the valley and in the low places. The number of pond walls to be constructed depends upon the slope of the land and drainage system to be provided. Barrage ponds should not be built where the flow of water is too great; it is difficult to keep the water from breaking down the wall, if the pressure of water is too great. Even when the flow of water is not great, however, barrage ponds require over flow channels. Because barrage ponds are usually built in low areas they are likely to fill up with heavy rains. The over flow channels discharge the extra water away form the ponds. If this extra water is not taken out, the pond wall may break. Advantage: The cost of construction is less. Disadvantages  Difficulty to manage.  Difficulty to control disease.  Difficulty to prevent the entry of unwanted things. Diversion ponds These ponds are made by bringing water (diverting) from another source like stream or river. Channels are dug to carry the water from the source to the pond. Diversion ponds can be made in a number of ways. Sometimes a pond is dug in flat ground can be made by slightly enlarging a natural depression of the land. Number of