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

[razmjooeis at faunalia.co.uk]

Hi Josh

In replay to your 2nd question, there was an interesting presentation by
John Abbott of OS back in November last year in Newcastle University on
OpenData
http://www.ncl.ac.uk/cegs.cpd/cpd/osworkshopoutline.php
The presentation from him is not available....
They provide some sort of service similar to OpenLayers, so you can build
your webmapping without any need to download and serve the data. It is
very handy as it has their standard symbology and copyright logo, etc.
http://oobrien.com/vis/bikes/

For the first part, can't you save the color table from one raster and
apply it to the patched one...there might be some missing values though.

Cheers
Sab

> 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.
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