[GRASS-user] Colour problems using r.patch (with OS OpenData)

[Joshua Arnott]

Hi grass-users,

I'm having some difficultly patching together some raster layers (OS 
OpenData District Raster - TIFF). Individually, the tiles display 
correctly using d.rast. However, if I patch them into a single tile 
using r.patch, the colour for all but the first becomes mangled. Example 
patching two layers (left and right):

The only reference on the UG I've found is thread:
http://lists.osgeo.org/pipermail/grass-user/1998-January/001017.html

The issue seems to be something to do with colour tables in the TIFFs 
being different. I've not tried the solution provided, as it appears 
clunky and I was hoping some progress had been made since '98!

r.in.wms seems to do something like this, patching together tiles 
downloaded from a WMS server. I've taken a look at the code for this, 
but found it difficult to pick out what is relevant. There is some 
mention to using r.mapcalc in a similar way, and also to the GDAL script 
gdal_merge.py, which I've not managed to get anything sensible from.

Ideally I'd like to be able to handle these layers from within GRASS 
(using pre-existing commands, or a python script) rather than externally 
(like gdal_merge.py seems to do), although at this point I'll take 
whatever I can get. Can anyone suggest a way forward in this?

Also, as a more general question, does anyone have experience using the 
OS OpenData dataset? I'm curious to see what people have done with it 
since its release.

Thanks in advance,
Josh.