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

[WolfgangZ]

Hi,

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?

Kind regards
Wolfgang

PS: I'm sorry for being a bit off topic, as this is not mainly grass 
related.