Before starting the digitalization, it is necessary to define the relation between the map picture and reality, i.e. to determine the transformation key used to transform the coordinates of the digitalized points to the target coordinate system.
The program uses the so called affinity transformation of coordinates. In this transformation, two different scale coefficients are used for the direction of the X and Y axes to better capture the unequal deformation of the map source in the longitudinal and transverse directions. The transformation parameters are determined using the least squares method (with the condition to minimize the sum of the square of the coordinate's adjustments on the identical points).
The minimum required number of identical points, thanks to the type of transformation used, is three (the transformation has five parameters: two translations, two scale coefficients, and rotation).
The maximum number of identical points is limited only by the capacity of the operational memory.