Planimeter working principle
1) When the tracing point is moved around an irregular boundary (only to be rotated clockwise) of the plan in the map, then the wheel is rotated.
2) The axis of the wheel is parallel to the tracing arm, and the plane of the wheel is perpendicular to the tracing arm, hence only those movement in which tracing arm moves perpendicular to its own axis the wheel is rotated, other times the wheel only slides.
3) The complete revolution of the wheels are read on the disc.
4) The drum is divided into 100 divisions and hence gives a reading between 1 to 100.
5) A Vernier caliper attached to the drum gives 1/10th of the drum reading.
6) A planimeter reading has 4 digits
Planimeter calculation formula
1.The anchor point is placed either inside the area to be measured or outside the area to be measured.
2.The area is calculated from the formula:
A = area measured
M = Multiplying constant
F = Final reading, I = Initial reading
N = Number of time 0 of the disc has crossed the index. (+ if 0 crossed clockwise, – if 0 crossed anticlockwise)
C = Constant which when multiplied by M gives area of zero circle. C is to be added only when anchor is inside the area. (Zero circle is obtained when the tracing arm moves in such a way that the wheel does not rotate but only slide, this generally happens when the tracing arm makes 90 degrees with the anchor arm while tracing a circle).