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

GRASS GRASS user
1

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.

2

Hello Wolfgang,
I fear the image you point is lightly shaded, so the color table won't fit the elevation model (hope I'm wrong...)

VB

WolfgangZ a écrit :

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.

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3

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@gclements.plus.com>

4

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?

As this is a WMS server, maybe r.in.wms's "list layers" option shows some
elevation options? Worth a look..

Otherwise, it's possible, but you'd have to do a fair bit of work which
may not be worth the effort.

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

IHO interesting problems are not OT if grass can be the tool to solve
them..

Hamish

5

Hi,

only to keep you updated: I compared one layer of the map server with SRTM data from that region. Some tendencies are recogniseable, but the scatter is too big to get something reasonable. So it seems not really possible to extract elevation data ;-((

regards
Wolfgang