Wheeled Planimeter Map area measurement tool
Images courtesy of Heather McPherson of Moray and Aberdeenshire Forest District.
Planimeter map area measuring device used up until about 2000 - 2001.
This planimeter consists of a little wheeled body which incorporates a counter driven by the one of the special wheels which turned by friction as it was dragged across the map. The wheeled body is fixed on one moveable tracer arm at the end of which is a very fine point (under a magnifying glass) allowing the user to carefully follow the closed boundary of the area to be measured. The second arm is detachable and at one end a ball pin fits into a socket in the wheeled body. The outer end of this second arm incorporates a small sharp spike which is pinned to the map and acts as an anchor around which the body can wheel and move easily as the user starts following the boundary lines of the areas to be measured with the tracer arm point.
It was a very cleverly engineered bit of kit and with careful use, uniform map surfaces and providing several readings were taken – usually three – good results could be obtained. The general aim was to try and measure the large forest area first, at least several hundred acres, and then measure the individual compartments that made up the area hoping to arrive at the same total area at the end of the process!
The planimeter could be adjusted to measure areas on different map scales and for this purpose each machine had a set of recorded tracer arm vernier scale readings (shown in the PDF) which was set up by moving the wheeled body along the tracer arm by a knurled screw once locking nuts were loosened.
Note that on this machine the tracer arm reading has been set at 139.3 which does not correspond to any of the recorded settings and may have been moved when it was stored away out of use. The counter needle in the body is also a little out of place and should be just approaching the zero from the left according to the reading on the vernier scale beside it showing 78.3
See the PDF for more close up views of the planimeter and it’s equipment, scale settings and box.
Download Large Version (1.5MB)
Groups: Tools, equipment and clothing