Polar planimeter
Also called: Planimeter · Topic: Maps and Map Scale · NCERT: Class 11, Ch 1 "Introduction to Maps"
Meaning
A polar planimeter is an instrument for measuring area on a map. You move its tracing arm clockwise around the boundary of the shape until you come back to the starting point. You note the dial reading before and after tracing. Take the difference between the two readings and multiply it by the instrument's constant (a fixed number for that instrument). The result is the area. It is more exact than counting squares on graph paper.
Example
To find the area of Chilika Lake in Odisha on a toposheet, a surveyor traces the lake's shoreline clockwise with the tracing arm. The change in the dial reading, multiplied by the constant, gives the lake's area on the map. The map scale then turns this into ground area.
Don't confuse with
- Rotameter: it measures the length of a winding line, not the area of a shape.
- Method of squares: it counts squares on graph paper instead of using an instrument, and its answer is rougher.