Friday, May 24, 2019

Steps, Technical Norms and Specifications for Professional Topographic Survey

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Topographic Survey

1. Conduct reconnaissance survey of the given area then formulate major traverse framework which must be enclosed the given area then formulate the minor traverse frame work within or outside the major frame work. Prepare an overall sketch in A2 size paper forming a closed minor traverse around the periphery of the area by establishing the traverse stations. While selecting the traverse stations, maintain the ratio of maximum traverse leg distance to minimum traverse leg distance less than 2:1 for major traverse and 3:1 for minor traverse

2. Referencing of each traverse station is utmost necessary and should be shown in sketch

3. Horizontal Control and Vertical Control of the given area must be conducted based upon the given Standard Horizontal Datum and Vertical Datum. Coordinate must be transferred to the instrument station from the National Grid System by observing to at least 3 known points (known coordinates). Resection station should be selected in the system of major framework. If it is not possible, resection station must be connected by making loop to the nearest Major traverse Station

4. Station pegs should be marked as 14Mı for Major Traverse and 14m1 for minor traverse Prefix number 14 is camp group number; letter M or m denotes the major or minor traverse and the suffix number 1 denote the traverse station number. Use the same notation in plotting except for common station. (Use CP1 and CP2 for common station).

5. Measured two way traverse leg distance (i.e. in the forward and backward directions) by means of tape calibrated against the standard length provided in the field or by total station. Note that the precision of tape measurements should be >= 1:2000 and in case measured by Total Station, the precision should be >=1:5000 6. Measure two sets of horizontal circle reading of major traverse station either by Theodolite or by Total Station. The difference between face left and face right reading is not  allowed more than ±180°±120"(twenty seconds). Note that the difference between angles of two sets reading should be within a minute

7. Measure only one set of angles of minor traverse. The difference between face left and face right reading is not allowed more than ±180°±120". Precision on forward and backward distance measurements by tape should be >= 1:1000 and >=1:3000 in case of distance measured by Total station.

8. Balance the traverse by proper way adjusting angular and linear closing errors. The sum of interior/exterior angles in a closed traverse should be equal to (2N±4) x 90°. The permissible angular error for the sum of interior angles of the Traverse should be less than ±30"(n)^0.5 for major traverse and ±1'(n)^0.5, in for minor traverse respectively. Relative precision or total error of closure should be less than 1:5000 for major traverse and 1:3000 for minor traverse respectively

9. For Vertical Control, perform two peg tests before the start of fly levelling. Perform fly levelling to transfer R.L. from the given B.M. to the allocated minor traverse. Take three wire staff readings and establish other TBMs as per field requirement. Note that collimation error should be less than 1:10000. To eliminate collimation error, levelling instrument be set up in such position so that fore sight and back sight distances are equal. B.M and its R.L. will be provided in the field. The permissible error of closure for fly leveling is ±25(k^0.5) in mm.

10. Plot the major traverse and minor traverse at site by coordinate method in scale 1:1000 and 1:500 respectively. Plotting of minor traverse should depict the major traverse. Orientation check will be performed at site.

11. Carry out the detail survey of the given sub area (minor) either by tacheometric system using theodolite or by total station with reference to the major and minor traverse stations. Take the detailed sketch of the minor area including natural and man made features.

12. Never drive wooden pegs at the frequent public walkway. Station pegs should be driven flushed completely on the ground surface to avoid walking disturbance as far as possible

13. All the details are to be plotted with respect to vertical and horizontal control from the respective station. Use conventional symbols for plotting the map.

14. Topographic map of the given minor area should depict contour lines at suitable interval (contour interval = 1m). Interpolate the index contour lines precisely by arithmetic calculation method with the help of guide points and then interpolate remaining contour lines either by graphical method or by estimation method. Do not erase those guide points from the original sheet even after plotting of the contours.

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