COMPUTING AREAS FROM MAPS

If map of an area is available its area can be found by the following methods:

(i) Approximate methods

(ii) Using planimeter.

Approximate Methods

The following three approximate methods are available for calculating area from the map:

(a) Give and take method

(b) Subdivisions into squares

(c) Subdivisions into rectangles.

(a) Give and take method: In this method irregular boundary is approximated with straight lines such that area taken in is equal to the area given out. Accuracy depends upon the judging capacity of the engineer. Then the area with straight edges is divided into a set of simple figures, like triangles and trapezoids and the area of map is found using standard expressions.

(b) Subdivisions into squares: Similar to a graph sheet, squares are marked on a transparent tracing sheet, each square representing a known area. Full squares are counted. Fractional squares are counted by give and take approximation. Then the number of squares multiplied by area of each square gives the area of the map

 

(c) Subdivisions into rectangles: In this method, a set of parallel lines are drawn at equal spacing on a transparent paper. Then that sheet is placed over the map and slightly rotated till two parallel lines touch the edges of the tap. Then equalising perpendiculars are drawn between the consecutive parallel line. Thus given area is converted to equivalent set of rectangles and then area is calculated