Affine transformation is most frequently used transformation of map sheets. This method is not suitable for old maps due to the introduction of one average key for the entire map sheet, which will not take into account local differences in different parts of the map sheet. This transformation in parts divides transformed area into smaller units, and for each unit their own key is calculated.
Correct distribution of control points is important for good result. Control points have to be chosen so to cover the whole area of interest. The triangular grid should be as far as possible equilateral. We have got a chance to define the envelop of the triangular grid with polygon to eliminate sharp triangles.
Sometimes we proceed in that way that first we make simple similarity or affine transformations of the map and then it is easier to find ground control points for transformation in parts.