Lecture 6_2024.pdf

UNIVERSITY OF MINES AND TECHNOLOGY, TARKWA FACULTY OF MINERAL RESOURCES TECHNOLOGY GEOMATIC ENGINEERING DEPARTMENT SOME METHODS OF LOCATING SOUNDINGS Lecturer 6 Cynthia B. Boye (PhD) July 2024  Learning Objective...

UNIVERSITY OF MINES AND TECHNOLOGY, TARKWA FACULTY OF MINERAL RESOURCES TECHNOLOGY GEOMATIC ENGINEERING DEPARTMENT SOME METHODS OF LOCATING SOUNDINGS Lecturer 6 Cynthia B. Boye (PhD) July 2024  Learning Objectives ❖ It is expected at the end of this lecture that students should be able to acquire knowledge on some methods of locating soundings. The following would also be considered: ❖ Observations entirely from the shore. ❖ Observations entirely from the sounding boat. ❖ Observations from both the shore and the sounding boat.  Methods of Locating Soundings Depending on the situations of observation, the following methods are adopted to locate the soundings. ❖ Observations entirely from the shore. ❖ Observations entirely from the sounding boat. ❖ Observations from both the shore and the sounding boat.  Observations Entirely from the Shore Observations entirely from the shore include the following methods: o By transit and stadia. o By range and one angle from the shore line. o By two angles from the shore line. Location by Transit and Stadia In this method, the following steps are involved (Figure 18): ▪ Establish a range line set up a transit on it. ▪ Hold a leveling staff vertically on the bottom of the sounding boat. ▪ Row the boat along the range line. ▪ Take stadia readings at the instant of taking soundings. ▪ Compute the distances by using tacheometric formulae. A’ A a1 a2 a3 a4 B’ B b1 b2 b3 b4 c4 C’ C c1 c2 c3 D’ D d1 d2 d3 d4 E’ E e1 e2 e3 e4 G’ G g1 g2 g3 g4 F’ F f1 f2 f3 f4 H’ H h1 h2 h3 h4 Figure 18: Location by transit and stadia  Field Observations Divide the shore line into various range lines AA', BB', CC', etc. at equal interval. Set up a transit/theodolite at A and erect a prominent signal at A' Ask the boatman to row the boat direction A'A Take stadia reading on a vertically held staff in the boat when it is at a1. Simultaneously, a sounding must be taken by sounding party of the boat. Row the boat to a2 and repeat the observations. Take observations on the range line A'A produced till stadia rod remains clearly visible by the transit man at A.  Field Observations ▪ Row the boat perpendicular to the range line A'A till it comes on the range line BB'. ▪ Take observations and soundings at b4, b3, b2, and b1 ▪ Similarly take soundings on the other range lines.  The following points may be noted: ▪ The transit should be set up near the water level so that horizontal sights may be taken on a stadia rod and thus vertical angles may be avoided. ▪ The transit may be set at any other point on the shore line whose position is predetermined and whose azimuth must be observed and recorded. ▪ The method is rapid and sufficiently accurate but it is suitable only in shallow and smooth waters. ▪ The method is unsuitable if the soundings are taken far from the shore line.  Location by Range and one angle from the shore line In this method, the following points are taken: ▪ Set up a number of range lines AA', BB', CC', etc. ▪ Set up the transit on range line BB', perpendicular to the range AA' at a distance X 1 meters. ▪ The boat is rowed along the range AA'. ▪ Clamp both the plates of the transit at zero and sight station A ▪ Loosening the upper plate, swing the telescope to boat. ▪ The signal man in the boat warns the instrument man to be ready by raising a flag a few seconds before sounding. ▪ The signal man lowers the flag at the instant of sounding.  A’ A a1 a2 a3 B’ B b1 b2 b3 b1 c2 C’ C c1 c3 D’ D d1 d2 d3 Figure 19: Range and one angle ▪ The transit man bisects the signal at the instant and records the angle ABa1 = θ1 correct to a minute. He should also record the time of observation in the field book. ▪ The boat is then rowed to be on range BB' and the transit is shifted to a point C, perpendicular to BB' at a distance X2 meters. ▪ Make necessary observations for various soundings as explained above.   Location by two angles from the shore line This method is applicable when it is impossible to keep the boat on a fixed range or where the shore topography does not permit laying out a system of intersection lines (Figure 20). The following steps are involved: Set up two transits at predetermined stations on the shore line which provide good visibility and intersections. Keeping both the plates of the transits clamped to zero, each transit man orients his transit to each other transit.  Loosening the upper plate, the observers follow the sounding rod of the boat with vertical cross hair. When a signal is given from the boat, both transit men must observe horizontal angles and time simultaneously. The recorder of the boat should also record the time of sounding.  A P1 P2 d B Figure 20: Two angles from shore line  Calculations Let A and B be the transit stations separated by a distance a distance d. α1 and α2 are horizontal angles BAP1 and ABP1 respectively where P1 is the location of the boat at the time of taking the sounding. Knowing the length AB and angles α1 and α2, the triangle ABP1 may be solved by applying the sine rule.  Calculations Knowing the distance and bearing of BA, the bearing of BA and AP1 can be computed and coordinates of the sounding location P1 may be obtained from the following relation (Figure 21).  P1 Path of the boat y A B x d  The following points may be noted. The lines from two instrument stations to the sounding location should intersect nearly at right angles. Both transit men should compare their watches with recorder before commence- ment of the day’s observations. Both the instrument men should check the orientation of their transits at frequent interval during day. The method has the disadvantage as it requires the transits and also much time is lost in occupying new station observation.  Observations Entirely from the Sounding Boat ✔ Observations entirely from the sounding boat include the following: ✔ Range and one angle from the boat. ✔ Two angles from the boat. Location by range and one angle from the boat In this method the angle at sounding between the ranges the fixed signal on the shore, line and is observed with a sextant from the boat (Figure 22). A’ B’ C’ D’ A B C D P1 P2 Figure 22: By range and angle from boat  Procedure The Following steps are involved: 1. Set up a number of range lines AA', BB', CC' at convenient locations. 2. To locate soundings along the range line AA' fix a signal at C’, the end of the range line CC'. 3. Row the boat along AA' and measure angle α1 at the instant of taking sounding and also note down time. 4. Proceed along AA' produced and make soundings at regular intervals. 5. Follow the above steps for locating soundings along other range lines. 6. Find the perpendicular distance (x) from the signal station to the range line AA'. 7. Compute the distances AP1, AP2, etc. equal to xcotα1, xcot α2, etc. The following points may be noted. ✔ This method is not generally used. ✔ The only advantage of the method is better control over the field work. B A C P Figure 23: Two angles from the boat side-scan sonar Procedure The following steps are involved. 1. Observed the angles APB and BPC simultaneously at the sounding P. 2. Calculate the coordinates of the point P in terms of those of the points A, B and C, by a well known three point problem. 3. Note: The following points may be noticed. 4. Ensure that the boat is not on the circumference of the circle passing through A, B and C, to avoid failure of fix i.e. total angle at P and total angle B should not sum up to180˚. 5. The precision of the location of a sounding varies with the relative locations of three known points A, B and C, as under: 6. If station A, B and C, are in a straight line or if B is nearer to the boat than A and C, the location of the sounding is precise unless one of the angles α1 and α2 is small. 7. If the sights PA, PB and PC are extremely long, the values of α1 and α2 will be small and consequently location will be weak. 8. The precision of location is poor when the sounding approaches the circle through the three points A, B and C.  Observations from both the Shore and Boat The Observations from both the shore and boat include the following methods: Range and time interval. Intersecting ranges. Distances along a stretched wire across a river.  Location by range and time interval The following steps are involved in this method (Figure 24). i. Set up a number of range lines AA', BB', CC' etc. at convenient places. ii. Start the boat from the shore and attain a uniform velocity before first sounding is taken. iii. Take sounding at α1 and note down the time of observation iv. Row the boat along the range line AA' and take sounding at regular interval of time. v. Knowing the speed of the boat, and the interval of time between succeeding soundings, the distance may be calculated. A' B' C' A B C α1 b1 c1 α2 b2 c2 α3 b3 c3 α4 b4 c4  The following points may be noted. i. The method is only used if better accuracy in determining the location of soundings is required. ii. This method is employed to obtain the soundings between soundings located by range and one angle either from the shore or from the boat. iii. This method is used in still water for short distances.  Location by Intersecting Ranges This method is generally used to determine changes in the bottom of a water body due to scouring or silting or to determine the quantity of material removed by dredging. To know these changes, it is necessary to repeat the sounding at the same points at regular interval (Figure 25).  A’ B’ C’ D’ A B C D b1 c1 d1 a1 b4 c4 d4 a4 Figure 25: Intersecting ranges  Procedure: The following steps are involved i. Range AA', BB', CC', DD' etc., are permanently marked by means of range signals at A, A', B, B', C, C', etc. ii. Row the boat to several intersections of the ranges and take soundings in the usual way. iii. To locate the sounding at a1 row the boat to the point where ranges AA' and BC' intersect. iv. Similarly, the sounding at b1 is located by the intersection of ranges BB' and D'C. The procedure is repeated for locating other soundings. The following points may be noted a. The signals fixed at the range lines should not be disturbed. b. The precision of the method depends upon the distances between the intersecting range lines.  Location by distances along a stretched wire across a river ✔ In this method, a wire is stretched between two fixed points on opposite banks of a narrow river, and marked by metal tags or cloth pieces at required interval along the wire. ✔ The distances of each tag may be calculated from a reference point, a zero station on the shore. ✔ The zero mark may be defined by a plumb bob (Figure 26).  Zero station Tags Stretched wires Figure 26: Distances along a stretched wire ✔ The boat is rowed to the points under each tag and soundings are taken. ✔ This is most accurate and expensive method. ✔ This method can be used to survey the depth contours (fathom lines) of a river bed. ✔ You may run a theodolite traverse along one of the banks and establish a number of points at 30 m to 40 m apart. ✔ The soundings are then taken along the sections across the river in suitable directions preferably perpendicular to the control points (Figure 27). ✔ Plot the traverse points sections and depths of the points determined by sounding on a paper on desired scale. ✔ Interpolation of various submarine contours may be made as in the case of contouring by spot levels.  D’ D C’ C B B’ A A’ 2m 4m 2m 4m Figure 27: Bathymetric contours of river bed Thanks for your attention Thanks for your attention Bye for now!