Approximate Methods

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

(1) Give and take method           

(2) Subdivisions into squares

(3) Subdivisions into rectangles.

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

(b) Subdivisions into squares: squares are marked on a transparent tracing sheet, Similar to a graph sheet, each square representing called area. Full squares are counted. By give and take approximation Fractional squares are counted. Then by area of each square the number of squares multiplied gives the area of the map given figure shows such like scheme. Finer the mesh better is the accuracy.

(c) Subdivisions into rectangles: a set of parallel lines are drawn at equal spacing on a transparent paper in this method. Then that sheet is placed over the map and slightly rotated until 2 parallel lines touch the edges of the tap. Then between the consecutive parallel lines equalising perpendiculars are drawn. Thus given area is converted to corresponding set of rectangles and then area is calculated as shown in figure.