A relief map shows the height of the land above the mean sea level on a flat surface. Several methods have been developed to show the relief features on a map. Some of them are the contours, form lines, layer- colouring, hill-shading, spot heights, bench marks, hachure’s, etc. These methods have their merits and demerits. Sometimes a combination of these methods is adopted, such as contours and form lines, contours and layer-colouring, etc.
1. Contours:
A contour is an imaginary line of constant height above the mean sea level. The difference between the values of any two successive contours is known as the contour interval, or the vertical interval (V.I.). The horizontal distance between any two contours is called the horizontal equivalent or H.E. Its value depends upon the slope of the land. The contours are drawn in brown colour on the topo sheet.
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2. Form lines:
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These are approximate contour lines, based on general observation. They show approximate heights of the place. They are shown by broken lines in brown colour. The form lines help in finding the minor details of topography. These lines are not numbered.
3. Spot Height:
It shows the exact height of a place above the mean sea level on the map. It is shown as a dot in black colour, followed by a number, which is the height of that point, such as 1040. These are plotted on the map after actual survey. The spot height does not give any idea about the relief.
4. Bench Mark:
These are marks shown on prominent places, such as rocks or buildings in the field. They indicate the actual height, so measured by the surveys. It is marked on the map as BM120. The bench marks serve as points of reference for other places.
5. Hachures:
These are finely drawn disconnected straight lines and indicate the direction of water flow. They are drawn along the direction of the maximum slope. The hachure lines are thicker and drawn closely on the steep slopes and are thin and wide apart on the gentle slope.