## transformation in parts

bad distribution of control points

there are no points in middle part

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.

better distribution of control points,

but in left part the tringles

are not optimal

better distribution of control points,

the envelop is defined by polygon

and the triangles are better

(see left part)