[GRASS-user] patching maps together

[Hamish]

Tim wrote:
> I am trying to patch together a number of 1 km by 1 km tiles of a map
> whose values are either 0 or 1 (black on white background).
> 
> If I use r.patch to put 2 tiles together is appears to work fine but
> if however I try and patch more, even if it is two tiles of already
> patched tiles I get one set when patched as black background and
> white features and the other part as white background and black
> features.  It appears that the second in the case of already patched
> data or third and subsequent in case of more than 2 tiles is inverted
> with 1 becoming 0 and 0 becoming 1.
> 
> I have tried reducing region to the smallest.  Maps are at 0.2m
> resolution.  I am hoping to get a 5 tile by 5 tile block
> 
> I have tried this across two different platforms (Mac OS and OpenSuse
> ) on the latest downloadable binary versions of GRASS.
> 
> Am I doing something wrong?


I suspect that the images are paletted, and the color palettes differ
between the images. After confirming that you might run the images
through a bulk color palette -> 24bit RGB filter. (I'm not sure what the
right NetPBM or ImageMagick command is, but I'm sure there is one)

after that, gdalbuildvrt is wonderful for import + patch in one step.


good luck,

-- 
Hamish <hamish.webmail at gmail.com>