[gdal-dev] Warping an already georeferenced image with control points

[Jan Hartmann]

Hi,

I am trying to georeference old maps in two steps: first georeferencing 
them from the old projection to a modern one, and then ameliorating the 
georeferenced image by rubber-sheeting it on the base of control points, 
mostly triangulation points for which the coordinates in the old and new 
maps are known.

The second step causes trouble: if I add  control points to the 
georeferenced image with gdal_translate -gcp , the projection 
information gets lost.

Yet, AFAICS it should be possible: if I use gdaltransform -s_srs 
EPSG:28992 -t_srs EPSG:28992 -gcp ... , I can use control points with 
coordinates in EPSG:28992 and get a correct transformation.

So: regarding this behavior  of gdal_translate, to discard projection 
information when control points are added: is it a bug or is it the 
expected behavior? In the last case, how can I rubber-sheet an already 
georeferenced map?

Jan