[GRASS-user] OT: converting color graphic to height data possible?

[Glynn Clements]

WolfgangZ wrote:

> I found a layer on a web map server that shows elevation data (no shaded 
> relief as I think). Is it possible to convert the color information that 
> is in the image back to elevations? Here a link to one image:
> http://mapas.topografia.upm.es/cgi-bin/gtopo?SERVICE=WMS&VERSION=1.1.1&REQUEST=GETMAP&LAYERS=spain25mtintashipsometricas&SRS=EPSG:23030&BBOX=535900,4779200,545900,4789200&WIDTH=200&HEIGHT=200&FORMAT=image/tiff
> 
> I suppose r.mapcalc would be the tool to use, but it seems not possible 
> without the right color map. Any suggestions?

First, you need to determine whether it's relief-shaded or if there's
a direct relationship between the colour and the elevation.

If it's the latter, the next step would be to convert it to a
single-channel image, either grey-scale or indexed-colour using the
correct palette ordering. That's the hard part.

I suspect that you would be better off just grabbing the appropriate
tile from the GTOPO30 dataset (given the name, there's a good chance
that's where it came from in the first place).

-- 
Glynn Clements <glynn at gclements.plus.com>