Once the known area is traced, you can see how many revolutions the dial scale indicates. The known area divided by the dial reading equals the constant. Once the constant is determined, the measuring dial is reset and the boundary is traced by moving the magnifier over the boundary in a clockwise direction. The reading on the dial is multiplied by the constant to give the desired area of the plot. This constant can be used so long as the length of the pole arm is not changed.
Digital planimeters require initial settings for units and scale if they are so equipped.
There is no need to determine a constant when using any digital planimeter. Best results are obtained by measuring over smooth, but not glossy surfaces. Photos or laminated maps increase the chances of the measuring wheel slipping, which will result in inaccurate measurements. Folds, seams, tape, or any irregularity on the surface can also cause problems. An adjustable tracer arm allows use on a variety of different maps.