No, this doesn't seem to work. Although the coordinate system *is*
retained when I add -a_srs to the gcps, the boundaries of the input file
are discarded: they are reset to the pixel dimensions of the file. Is
this a bug?
Jan
Frank Warmerdam wrote:
Jan Hartmann wrote:
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,
If you use the -a_srs switch of gdal_translate in combination with the
-gcp switch to add gcps, the GCPs should be recorded as being in the
coordinate system described by -a_srs. Because there is no reason to
believe that the current image coordinate system is related to the
coordinate system of the GCPs, no effort is made to use the existing
image
coordinate system as the default GCP coordinate system.
By the way, setting GCPs with a GCP coordinate system on an image is
likely
to not work with most GDAL supported formats, and the coordinate system
related aspect may also be supported in a variable fashion depending
on the
format driver's implementation.
Hopefully things work well with the GeoTIFF format, which I assume you
have
been using.
Best regards,
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