Levelling Guide PDF - JDT ISLAM POLYTECHNIC COLLEGE

MODULE III LEVELLING Levelling is the art of finding the relative heights and depths of the objects on the surface of the earth. It deals with measurements in a vertical plane. Level surface: A level surface is defined as a curved surface which at each point is perpendicular to the direction of gr...

MODULE III LEVELLING Levelling is the art of finding the relative heights and depths of the objects on the surface of the earth. It deals with measurements in a vertical plane. Level surface: A level surface is defined as a curved surface which at each point is perpendicular to the direction of gravity at that point. Level line: Level line is a line lying in a level surface. Horizontal surface: It is a plane which is tangential to the level surface. The line lying in the horizontal surface is called horizontal line. Datum: It is an imaginary level surface or line from which vertical distance of the point above or below this lines are measured. In India the datum adopted for great trignometrical survey (GTS) of Indian department is mean sea level(MSL) at Karachi. Reduced level(Elevation): It is the height or depth of that point above or below any datum. Bench mark: It is a fixed reference point of known elevation(RL). Types are 1. G.T.S bench mark: These are established by the great trignometrical survey of Indian department. These have been set up at intervals all over the country with very high precision with reference to the MSL at Karachi as the datum. 2. Permanent bench mark: The GTS bench marks are located far away from one another. To facilitate survey work, Government agencies such as PWD have established bench marks between GTS bench marks at closer interval. These are called permanent bench mark. 3. Arbitrary bench mark: In small leveling operations, the RL of a well-defined reference point is assumed. These are called arbitrary bench mark. 4. Temporary bench mark: These are the reference points, which are established when there is a break in the work. That is the all work cannot be completed at a stretch. Types of leveling Instrument A leveling instrument essentially consists of 1. A leveling head with three foot screws which helps to bring the bubble in central. 2. A telescope that provides line of sight. 3. A bubble tube to make the line of sight horizontal. 4. A tripod for supporting the leveling instrument. Components of Dumpy Level Dumpy level consists the following parts or components Telescope Bubble tubes Vertical spindle Tribrach screws Foot screws Leveling head JDT ISLAM POLYTECHNIC COLLEGE Page 1 TELESCOPE Telescope is used to observe the distant object through line of sight provided by its arrangement. In general, the telescope is fixed to the vertical spindle of dumpy level so that it can be rotated along with vertical spindle. Parts of Telescope in Dumpy Level The important parts of telescope are as follows Eye piece Objective lens Diaphragm Focusing screw Ray shade EYE PIECE: Eye piece is used by the observer’s eye to view the distant object. It contains magnifying glass which magnify the observing image and also the cross hairs of diaphragm. So, accurate reading can be obtained. Erecting eyepiece is used to view the normal image which is generally inverted by objective lens. OBJECTIVE LENS: Objective lens is provided at the other end of the telescope. The objective lens consists of two parts, the front part consists convex type lens and the back part consists concave lens. So, the image obtained from the objective lens is always inverted. DIAPHRAGM: Diaphragm is provided in front of the eye piece. It contains cross hairs made of dark metal which are arranged in perfect perpendicular positions. These cross hairs are used by the eye piece to bisect the objective through objective lens. FOCUSING SCREW: Focusing screw is used to adjust the focus if cross hairs and the image clarity. The magnification of eye piece is managed by this focusing screw. BUBBLE TUBES: Bubble tubes are provided to check the level of the instrument. Two bubble tubes are provided in a dumpy level which is arranged perpendicular to each other on the top of the JDT ISLAM POLYTECHNIC COLLEGE Page 2 telescope. The instrument is said to be in perfect position when both the bubbles of the tubes are at center or middle of the tube. VERTICAL SPINDLE: Vertical spindle is located at the center of the whole instrument. The telescope can be rotated in horizontal direction with respect to vertical spindle. The instrument is connected to the tripod stand using vertical spindle. TRIBRACH: Tribrach plate is parallel to the leveling head or trivet. It is connected to trivet by leveling screws or foot screws which can adjust the tribrach plate. The horizontal level of the instrument can be achieved by adjusting this tribrach plate. FOOT SCREWS: Foot screws are provided to regulate the tribrach position and hence the instrument can be leveled which is known by observing the bubble tube. The tribrach plates can be lowered or raised using foot screws. The position of tribrach is said to be correct when the bubble in bubble tube is at center. LEVELLING HEAD: Leveling head is also called as trivet. It contains two triangular shaped plates which are arranged parallel to each other. Three groves are provided at the three corners of the plates in which foot screws are supported. TYPES OF LEVEL 1. Dumpy Level: It has the vertical spindle and the telescope barrel cast in one piece and therefore is termed as solid dumpy. It is simple, compact and stable. The telescope being rigidly fixed to its supports can neither be rotated about its longitudinal axis nor can it be remove from its supports. 2. Wye level: In case of Wye level the telescope is carried in two vertical wye supports. The wye supports consists of curved clips. If the clips are raised the telescope can be rotated in the wyes or removed and turned end for end. 3. Modern Tilting Level: The telescope of this level has a small motion about a horizontal axis just below it. In these the telescope along with its bubble tube can be levelling by a tilting screw without using the foot screws of the instrument. 4. Automatic Level: it is leveled automatically. This is achieved by using a tilting compensator suspended like a pendulum inserted in the path of light rays. These have high precision, high speed and operational comfort. LEVELLING STAFF It is a graduated rod used for measuring the vertical distances of the points above or below the horizontal line of sight. Types are 1. Sop with telescopic staff: It is commonly used type of leveling staff. It is usually arrange in three telescopic lengths and is 4m long when fully extended. Top length of 1.25m,slide into the central box of 1.25m and a bottom box of 1.5m length. The smallest division on this staff is 0.005m. 2. Folding metric staff: It has 4m length arranged in two lengths each being 2m and connected at the middle. It can be folded and the two pieces can be detached from each other, when required to facilitate easy handling and using one piece. JDT ISLAM POLYTECHNIC COLLEGE Page 3  3. Aluminium telescopic staff : It is made of special aluminium alloy and is anticorrosive. It has a specially designed easy locking system by pressing the push button. It is provided with a canvas bag so that it can be kept covered when not in use. SOME DEFINITIONS 1. Line of collimation: It is an imaginary line joining the intersection of cross hairs at diaphragm to the optical centre of the object glass and its continuation. It is also called line of sight. 2. Axis of telescope: It is an imaginary line joining the centre of the eyepiece and the optical centre of the object glass 3. Axis of bubble tube: It is an imaginary line tangential to the bubble tube at its middle point. 4. Vertical axis: It is the axis perpendicular to the line of collimation or line of sight. 5. Back sight (BS): It is a staff reading taken on a point of known reduced level as on a bench mark. It is the first staff reading taken after the level is set up and leveled. It is considered to be positive. 6. Fore sight (FS): It is a staff reading taken on a point whose reduced level is to be determined. It is the last staff reading taken before shifting of the level to another position. It is considered to be negative. 7. Intermediate sight(IS): It is any reading other than BS and FS taken on appoint of unknown reduced level. That is all reading taken in between BS and FS are called IS. 8. Height of instrument(HI): It is the reduced level of the line of collimation when the instrument is correctly leveled. That is the height from the datum to the line of sight. 9. Staff Station: Any point where the staff is held and the reading is taken during leveling. 10. Change Point(CP): It is an intermediate staff station on which two sights FS and BS are taken and is used for the purpose of changing the position of the instrument. 11. Focussing: It means to set the eye piece and the object glass at the proper distance apart for the clear vision of the object sighted. 12. Parallax: It is a condition arising when the image formed by the objective is not in the plane of the cross hairs. It can be eliminated by proper focussing of eye piece and object glass. Adjustments of level Two types are 1. Temporary adjustments 2. Permenant adjustments Permanent adjustments This adjustment should be permanent for a long time depending on the type of instrument. 1. The axis of bubble tube should be perpendicular to the vertical axis. 2. The line of sight should be parallel to the axis of bubble tube. 3. The horizontal cross hair should be in a plane perpendicular to the vertical axis. Temporary adjustments(working operations of a Level) It is made at each set up of the instrument before starting to take readings. These are 1. Fixing the instrument on stand. JDT ISLAM POLYTECHNIC COLLEGE Page 4  2. Levelling up of the instrument 3. Focussing Fixing: The level is fixed on the tripod stand by turning round the lower part. Then screw the instrument. Levelling up: At first the instrument is leveled using their legs. Then level it by using foot screws ie, Place the bubble tube parallel to any pair of the foot screws and bring the bubble to the centre of its run by turning the screws equally either both inwards or both outwards. Then turn the third foot screw to bring the cross bubble to its central position. Repeat this until both the bubbles are centred. Focussing: Focussing are done by two steps:- 1. Focussing the eyepiece: Point the telescope towards the sky or hold a piece of white paper in front of the telescope. Move the eye piece until a distinct and sharp black image of cross hairs is seen. 2. Focussing the object glass: Direct the telescope towards the staff and turn the focussing screw until a clear and sharp of staff graduations is obtain. 3. The horizontal cross hair should be in a plane perpendicular to the vertical axis. Field note on leveling (Level book) The leveling field notes are entered in a note book called a leveling book. The pages of a level book are printed in a tabular column so as to facilitate booking of reading and reduction of levels (finding RL). There are two methods to find out RL of point from staff reading taken in a field. 1. Height of instrument method STATION BS IS FS HI RL REMARKS 2. Rise and fall method STATION BS IS FS RISE FALL RL REMARKS Height of instrument method In this method HI is obtained at every set up by adding BS to the RL of the BM. To start with, HI is calculated by adding the BS with the RL of the starting point(BM). The RL of the intermediate points and change point are then obtained by subtracting the respective reading from HI. When instrument is shifted, the HI becomes changed and the other RL are finding out by subtracting the respective readings from these new HI. Then the process is repeated until all the RL of the pointe were taken. Then an arithmetic check is applied. ƸBS-ƸFS=Last RL-First RL, sum of BS-sum of FS=Last RL –First RL Rise and fall method In this method the difference between consecutive point is calculated by comparing each point with that immediately preceding it. The difference of their staff reading indicates rise or fall according as any staff reading is smaller or greater than that the preceding point. The RL of each point is finding out by adding rise or subtracting fall from the RL of the previous point. Arithmetic check is JDT ISLAM POLYTECHNIC COLLEGE Page 5 ƸBS-ƸFS= ƸRise-ƸFall=Last RL-First RL, sum of BS-sum of FS=sum of Rise-sum of fall=Last RL –First RL Difference between HI method and Rise and Fall method HI method Rise and fall method The method is simple and easy The method is complicated Reduction of levels is easy Reduction of levels take more time There is no check on RL of intermediate stations There is a complete check for all readings There are only two check There are three check for arithmetical accuracy ƸBS-ƸFS=Last RL-First RL ƸBS-ƸFS= ƸRise-ƸFall=Last RL-First RL It is used for longitudinal and cross levelling It is used for earthwork calculations Problems were done in note book. Errors in Levelling 1. Instrumental errors 1. Imperfect permanent adjustment: If the permanent adjustment of instrument is not correct the line of collimation will be inclined even though the bubble is centred. 2. Defective bubble tube: If there is any defect in bubble tube, the bubble may remain in centre even though the bubble axis is not horizontal. 3. Incorrect graduations of staff: When the staff is new one ,the graduations should be checked with a tape. 2. Personal errors 1. Careless leveling up of instrument 2. The bubble not being central at the time of taking reading 3. The error due to imperfect focussing. 4. The staff not being held vertical. 3. Natural errors 1. Curvature and refraction: The effect of curvature of earth is to cause the objects sighted lower than they actually are while that of refraction is to make them appear higher than the actually are. 2. Effect of sun and wind: If there is bright sun shine eon the staff, readings cannot be taken accurately. 3. High wind: High wind cause shaking of the tripod and instrument and it becomes difficult to keep the bubble centred. 4. Settlement of tripod and staff: If the soil is not firm, there is possibility for the settlement of tripod and staff. Curvature and Refraction For long sights, that is greater than or equal to 200m the effects of curvature of earth and refraction of line of sight shall have to be taken into consideration. Due to curvature, the level line falls away from the horizontal line and the vertical distance between the horizontal line and the level line represents the effect of curvature of earth.Here the observed reading is more than true reading. JDT ISLAM POLYTECHNIC COLLEGE Page 6  Correction for curvature C c =0.0785d2, where d is the distance in km and C c is in metre. True staff reading=Observed staff reading-0.0785d2 Effect of refraction: The air is denser near the earth. The ray of light from the staff to the instrument travels from a thinner medium to denser medium. So the line of sight does not remain horizontal but it bends downwards and the reading should be smaller than the true reading. Correction for refraction(C r )=(1/7)C c =0.0112d2 True staff reading=Observed staff reading+0.0112d2 Combined correction due to curvature and refraction =C=0.06728d2=C c -C r True staff reading=Observed staff reading-0.06728d2 Problems were done in note book. Distance to visible horizon: Equation and problems were done in note book. Sensitiveness of bubble tube The sensitiveness of bubble tube is defined as the angular value of one division marked on the tube. It is the amount the horizontal axis has to be tilted to cause the bubble to move from one graduation to another. Eg: If the tilting is 20’’ of arc when the bubble moves 2mm (one division), the sensitiveness of bubble tube is expressed as 20’’ per 2mm. A tube is said to be more sensitive if the bubble moves by more divisions for a given change in the angle. Sensitiveness of a bubble tube can be increased by 1. Increasing the internal radius of the tube 2. Increasing the diameter of the tube. 3. Increasing the length of the bubble 4. Decreasing the roughness of wall 5. Decreasing the viscosity of liquid. JDT ISLAM POLYTECHNIC COLLEGE Page 7