Saturday, August 3, 2013

Preparing a Topographical Map

So if you are a surveyor, then you might be asked to prepare a topographic map of the given area. As you know that a topographic map shows all the horizontal and vertical distances between two points you need to use the various Surveying instruments available with you. It is very nice if you have a total station, because it is a more accurate than then the other instruments, but if you are graduate from the Engineering institutes, where they taught you the basic methods of Surveying like, chaining, compass surveying, Theodolite traversing, Levelling, Plane Table, and Tacheometer, then it might be easier for you to use these methods to prepare your topographic map.
To prepare your topographic map you need to follow the principles of the surveying:
(1) work from whole to part
(2) Locate a point with respect to minimum a 2 numbers of already known points.
You can follow the following steps to prepare your map:
(1) Locate the control points very precisely and prepare the traverse joining these points:
 It can be done with a precise instruments like Theodolite and Chain. You need to find out the horizontal angles between the control lines using the Theodolite and the horizontal distances using Chain or Tape. You have to keep in mind while doing triangulation surveys it is important to keep the angles of a triangle in the range of 30 to 150 degrees. Equilateral triangle gives the best results.
(2) Locate the interior details: For the interior details of the area you use the control points as the reference points. So there are less chances of accumulation of the errors. You may need the plane table, Level, chain and other accessories to do so. If you use the plane table, then there is an advantage that you don't need to carry a measurement note book with you, because plotting is done at the same time on the field. While
 contouring: It is important to show the contours in a topographical map so you have to use the levelling techniques and the contouring methods to find out the points of same elevations and then joining them together. You can use a Auto-level and Levelling staff, Ranging Rods, Tape and other accessories to accomplish the field job. You can use the radial method in case of hilly areas and use the method of cross section in the plane areas. Now plotting them on the sheet is another job to be done when you have all the observations in your field note book. Do all the calculations to find out the R.L.(Reduced Level) of the point and draw them on the sheet.
Once points are transferred to sheet, you can use the methods of plotting the contours, like approximation method, arithmetic method or other graphical methods too.  You have to use the standard symbols to show the various features of the area on the map. There are different symbols for jungles, building, roads, cultivated area, electric lines, railway lines, cutting and filling etc. Contours are drawn free hand by joining the points of equal elevations and drawn in brown color. Elevations are written with the contour lines and a constant contour interval is followed.

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