It's similar in principle to the algorithm described here: http://www.mathsisfun.com/geometry/area-irregular-polygons.html http://www.mathsisfun.com/geometry/ar

It's similar in principle to the algorithm described here: http://www.mathsisfun.com/geometry/area-irregular-polygons.html http://www.mathsisfun.com/geometry/area-irregular-polygons.h... Prytz (hatchet) planimeters are more tricky and approximate in the way they work.