[gdal-dev] referencing scanned paper maps

[Oliver Eichler]

Hi,

I try to reference some scanned paper maps with UTM Zone33 projection and ED50 as datum. I use gdal_translate to add the GCPs and "+proj=utm +zone=33 +ellps=intl +units=m +no_defs". And then gdalwarp to actually transform the file. So far so good. 

However the datum shift for waypoints and tracks in longlat / WGS84 to UTM / ED50 has an error of ~100m. I can fix that with the correct +towgs84 parameter in the projection string.

Now I would like to add this information to the resulting GeoTiff. That's where my trouble starts. If I add +towgs84 to the projection string of the GCPs it seems to get purged by gdal_translate. Same if I add +wktext. And gdalwarp seems to ignore -t_srs parameter. 

Thus my question: How do I add the +towgs84 parameter to the file, to get it later on for WGS84 <-> ED50 conversion?

Oliver