[Fwd: Re: [gdal-dev] Warping an already georeferenced image with
j.l.h.hartmann at uva.nl
Wed Feb 11 07:47:16 EST 2009
Sorry to keep moaning about this, but I need an indication what's going
on here. Mind, I don't need an immediate solution: for the time being I
have a workaround. Just an idea whether this a real problem, a dumb
question, something that can be handled in the foreseeable future (or
not), perhaps with adequate funding. Everything is better than talking
to a blind wall.
Sorry again Frank,
Jan Hartmann wrote:
> 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?
> Frank Warmerdam wrote:
>> Jan Hartmann wrote:
>>> 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?
>> 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
>> coordinate system as the default GCP coordinate system.
>> By the way, setting GCPs with a GCP coordinate system on an image is
>> 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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