1.16.5. Methods of contouring

There are two main methods of surveying: (a) Direct method- This method finds the elevation of points in the area which lie on the selected contour line. The level instrument is set at a commanding position in the area and the elevation of line of collimation of instrument is determined. The required staff reading for a given contour line is then computed. The levelling staff is moved up and down in the area till the required staff reading is observed. The elevation of that point is marked on the plotted map. In this manner, all the points with same staff readings in the area are established, and in the process the instrument is also shifted to another station to cover more area. The same process is repeated for the next value of contour. This method is accurate but slow and tedious, hence suitable for small areas.

The direct method for contouring involves identifying the elevation of specific points along a contour line. First, a leveling instrument is placed at a location with a clear view of the area. The height of the instrument is established, which aids in determining the required staff readings. The leveling staff is adjusted to find the point that matches the desired elevation (the contour line). This involves moving the staff up or down until the correct reading is obtained. Recorded points at the same elevation are marked on the map, illustrating the contour line. This method is noted for its accuracy, but it is slow and time-consuming, making it best suited for smaller areas where detail is critical.

Imagine you're trying to find the height of a water level in a swimming pool. You have a measuring stick (the leveling staff) that tells you how deep the water is. You move the stick up and down until you find the point that matches a certain water height (contour line). Once you find that point, you make a note of it. You repeat this process at other locations in the pool, marking all the points where the water reaches the same height. Just like this, the direct method helps map out ground elevations accurately.

(b) Indirect methods- Here, RLs of some selected points (also known as spot levels) are taken. The contour lines are interpolated using the RLs of these selected points. For selecting the points, several approaches may be used: (i) Method of squares, (ii) Method of cross-section, and (iii) Radial line method. Out of the three approaches, first approach is most commonly used. In the method of square, the area is divided into a number of squares and all grid points are marked on the ground at regular interval. The grid interval is to be decided by the surveyor, depending on the terrain and scale of mapping. The RLs of all grid points are determined by levelling. The square grid is plotted on the drawing sheet, and RLs of grid points marked. Points of specific elevation value are drawn by linear interpolation method. These points are joined together with smooth contour line and its elevation value is written along the contour line. French curves may be used for drawing the smooth lines. To provide, more readability to contour values, every fifth contour line is made slightly thicker.

The indirect method of contouring utilizes selected reference points, known as spot levels, to infer elevations across a broader area. This method doesn't require measuring every point directly. Instead, surveyors take elevation readings at strategically chosen locations. The area is typically divided into squares, creating a grid where points are systematically sampled. Once these points are established, their elevations are recorded. Using linear interpolation, surveyors estimate the elevations between these points to draw contour lines. This method is generally faster than direct contouring and allows for the mapping of larger areas without losing significant detail.

Think of baking a cake. Instead of measuring the temperature of every part of the cake with a thermometer, you check the temperature in selected spots – the edges and the center. Then you determine how well-cooked the entire cake is based on these checks. Similarly, in indirect contouring, surveyors collect elevation data at chosen spots, enabling them to draw a detailed map without examining every inch of land.

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