9.8 Rectification of the photographed documents and maps

The above discussed method can be used not only to fit photographs taken onboard of aircrafts into map coordinate systems. When we take a picture about a document or a map sheet holding the camera in hand, the target is depicted by the same perspectic distortion. If we aim to reconstruct the original geometry of the planar target, or to rectify the photographed map in its own projection, we shall apply the method of this very chapter.

In case of text documents, it is – because of the difficulty of control point selection – not always easy. If needed, we can make slightly, by pencil, some small signs at pre-measured points of the documents for control points. After taking the photo, these signs should be removed without any damage of the original document. In case of photographed maps, this problem does not occur. The control points should be selected the same way we discussed in Chapter 6. The difference is that the rectification is to be done by the procedure discussed in this chapter. We usually don’t have any information about the vertical position of the photographed material. Thus the elevation of the control points are set to zero, as well as the elevation model pixel values. Applying this method, we can reconstruct the geometry of the photographed map and we can fit it to map coordinate system in the same algorithm.