[Qgis-user] Editing the world file

[Craig Leat]

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 Sadly, It has been withdrawn, but can perhaps be found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