Methods of Contouring

There are several methods of contouring, the final objective being the preparation of a contour map. The equipments required include those used for distance and elevation measurement. A chain or tape is used in ordinary contouring work for distance measurement and a level with a levelling staff is required for measurement of elevations. The following methods of contouring are commonly used, such as,

1. Direct Method of Contouring
2. Indirect Method of Contouring
   • Radial Line Method of Contouring
   • Grid Method of Contouring

1. Direct Method of Contouring

In the direct method of contouring, the desire contours are obtained by trial and error. It is exceptionally difficult, however precise methods of contouring. It is used when the extent of survey is small and greater accuracy is required in locating the contours.

The procedure for direct method of contouring are as follows:

• First it is to be ensure that a bench mark is available near the side of survey. If a bench mark is not available, then one should be located near the site by fly levelling.
• Once the benchmark is available, the level is setup at a suitable position, covering a large part of the area.
• A backside is taken on a benchmark and the height of instrument is calculated.
• From the height of instrument, the staff reading is calculated for the RL of desired contour.
• The levelling staff is held at various points on the ground until the desired staff reading is obtained.
• The point is suitably mark for theodolite or compass survey to locate it horizontally.
• A number of such points is obtained by similar trials on the desired contour. Over it the process is repeated for other desired contours.
• All the marked points are located in the plane from chain lines of a traverse or other suitable means.

2. Indirect Method of Contouring

In the indirect method of contouring, a general survey in conducted to obtain the vertical and horizontal distances of the points. In this methods of contouring when a large number of such points are available, the points of the required elevations are obtained by interpolation. Indirect methods are faster compared to direct method. The following indirect method of contouring are commonly used:

Radial Line Method of Contouring

The contouring small hilly areas, radial line method of contouring are run from the peak to cover the area. The guide points are taken on the radial lines and the elevations are calculated. The contour lines are then drawn by interpolation.

The following is the procedure for the radial line method of contouring:

• A benchmark is located near the peak by fly levelling from an available benchmark from a nearby site.
• Radial lines laid out from the peak conveniently to cover the area suitably.
• Staff readings are taken at some definite interval on each radial line and the elevations of the stations are determined.
• The contours of the required elevations are determined by interpolation.

Grid Method of Contouring

The grid method of contouring is used when the area is not very large and where the ground is not very much undulating.

The following procedure is adopted for the grid method of contouring:

• The area is dived into squares of regionable directions. Usually, 5m to 20m squares are adopted. The dimensions of the squares depend upon the nature of the ground and the number of the contour lines of required.
• The staff readings are taken at the corners of the squares only.
• Once the levels are available, the contours of different elevations are determined by interpolation.

So, here it is discussed about the different methods of contouring in surveying, such as, Direct Methods of Contouring and Indirect Methods of Contouring.

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