Cartographica Helvetica

Summary

Gustav Forstner:

Two methods for constructing distortion grids for the analysis of old maps

Cartographica Helvetica 18 (1998) 33–40

Summary:

Distortion grids are a means by which a general impression of the orientation and accuracy of old maps can be obtained. The concept of the geometric construction, or in other words, the construction using a standard scale of the distortion grids, placed emphasis on the appearance of the results rather than the ability to reproduce them. A completely objective construction of the grids is not possible since the choice of points used for a particular line, or even the elimination of inaccurate points, is subjective. Furthermore, a constant scale factor is usually lacking in old maps, and the distribution of points which can be identified is often very irregular. Especially in areas with few suitable points, the tendency to use distant points or to interpolate points over long stretches further distorts the results. These problems also arise in the computation of distortion grids, however not as strongly. Because these computations are usually automatic, it is important that the grids are defined only for areas with a sufficient number of points along the lines.

All grid lines can be influenced – not so much in their tendency, but in their details – by the free choice of parameters. Before additional analysis of data derived from such grids are carried out, the data should be checked up on its reliability to see if further conclusions may be drawn.