Mats Elfström wrote: > Hi! > A bit late in the day, perhaps but I'd like to recommend against manual > editing of world files and the like, unless you know exactly what you are > doing. > The way I see it, such files are intended to be written by software, and > read by software. > As has been pointed out here, the image file may hold its own georeference, > as is the case with geotiffs or ecw files. Editing the world file may lead > to unexpected conflicts and unpredictable results depending on which > information is used by default, or by some program option. I agree that this > priority should be user defineable, but not to deal with a situation where > the internal georeferencing and the world file are in conflict. This would > be an error condition and should be resolved as such. > One must also take into account the half pixel problem. The world file > references the centre of the northwest pixel of the image, whereas the > geotiff header references the northwest corner of the same pixel. In order > to be equal, the ground coordinate must differ by half a pixel in the world > file. Failure to observe this will give a noticeable error for coarse > resolutions in comparison with vector data. > Many years ago, Mentor Software released a free utility, the GeoTIFF > Examiner, which enables the user to edit and synchronize a world file and > the geotiff header. Sadly, It has been withdrawn, but can perhaps be found > in some remote corner of the Internet, Google and you may perhaps get lucky. > It's a most useful tool.
I face the situation where my national mapping authority distributes its raster products in the Lizardtech sid format, which for me is read only. I frequently encounter erroneous internal coordinates and need to fall back on the world file. As you say this is an error condition, but my mapping authority tends to be non-responsive regarding these issues because it works in Arcview, etc. I currently create vrt files manually to deal with this. On my to do list is a script that will create a vrt and update the coordinates from the world file. My life would be simpler if gdal would just use the world file ;) Regards, Craig _______________________________________________ Qgis-user mailing list Qgis-user@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/qgis-user
