Jan Hartmann wrote:
Now, this geotiff file with gcps is just an intermediate file. When
warping with control points, you need first to gdal_translate the
original image into a second file with gcps added, and than you can
gdalwarp that second file into the final map, a third file. I have
always wondered why this intermediate gcp-file was needed: you could
just as well transform directly from image to map, *if* gcps could be
indicated with gdalwarp, or am I overlooking something? Adding a -gcp
option to gdalwarp would AFAICS not be very difficult, it wouldn't break
existing applications, and it would make warping by control points more
efficient, because no intermediate files have to be generated any more.
I think I understand how the gcp option came to be added to
gdal_translate: some image providers, especially satellite images,
provide their images with control points within the file, so the -gcp
option has to be retained for gdal_translate.
So, even if I have been unable to make clear my exotic problems with
historical maps, could this feature, adding a -gcp parameter to
gdalwarp, be implemented with a reasonable amount of programming effort?
Jan,
I haven't been following this discussion as closely as I might. I think
you could gdal_translate to VRT format, instead of an intermediate GeoTIFF,
in order to add the GCPs in a VRT "wrapper" around the original GeoTIFF.
This will hopefully support GCP+SRS and geotransform+SRS.
I'm a bit hesitant to add the -gcp logic into gdalwarp as it already has
a very complicated interface, and because it would require wrapping the
input dataset in a VRT internally to accomplish the functionality without
overly complicating the rest of the calls into the library.
But, if the gdal_translate approach is unsuitable, I can ask Chaitanya to
implement that.
Best regards,
--
---------------------------------------+--------------------------------------
I set the clouds in motion - turn up | Frank Warmerdam, warmer...@pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush | Geospatial Programmer for Rent
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