[GRASS-user] shaded relief of srtm tiles

GRASS GRASS user
1

hello list,

i try to use r.shaded.relief on srtm tiles imported with r.in.srtm.
the results look nice, but adjacent tiles have a different average brightness.
I used the same parameters (altitude=44 azimuth=270 units=meters) for both maps and tried with grass versions 6.02 and 6.20
That's how it looks like for the original srtm tiles N42W005.hgt.zip
and N42W004.hgt.zip:
http://www.flickr.com/photos/h2guide/328146671/

What am i doing wrong? (Or what am i supposed to do in order to produce seamless shaded reliefs?)

Thanks in advance,
Thorsten

2

thorsten wrote:

hello list,

i try to use r.shaded.relief on srtm tiles imported with r.in.srtm.
the results look nice, but adjacent tiles have a different average
brightness.
I used the same parameters (altitude=44 azimuth=270 units=meters) for
both maps and tried with grass versions 6.02 and 6.20
That's how it looks like for the original srtm tiles N42W005.hgt.zip
and N42W004.hgt.zip:
http://www.flickr.com/photos/h2guide/328146671/

What am i doing wrong? (Or what am i supposed to do in order to produce
seamless shaded reliefs?)

try setting an absolute color table with r.colors,

or merge the two tiles first:

g.region rast=tile1,tile2
r.patch in=tile1,tile2 out=tile_merge

Hamish

3

thank you Hamish,

patching works fine ( http://www.flickr.com/photos/h2guide/329431459 ),
but i try to create a shaded relief of a hughe area,
so i will have to process it tile by tile.

what i am actually doing is:

r.in.srtm input=N42W005 output=tile r.in.srtm input=N42W004 output=tile
g.region rast=tile
r.colors map=tile rules=srtm
r.shaded.relief map=tile shadedmap=relief altitude=44 azimuth=270 units=meters
r.his h_map=tile i_map=relief r_map=final.red g_map=final.green b_map=final.blue
r.composite red=final.red green=final.green blue=final.blue levels=32 output=final;

"final" is the shaded and colored relief.

what do you mean by "absolute" color table?
i am using the predefined color table "srtm"
and the result is still different for the adjacent tiles:

http://www.flickr.com/photos/h2guide/329431463

pasting the colored maps (called "tile") together looks ok:
http://www.flickr.com/photos/h2guide/329472914

so the problem is the greyscale component (called "relief") produced by r.shaded.relief:

http://www.flickr.com/photos/h2guide/328146671

What should I do to get seamless tiles?

Thorsten

Hamish wrote:

thorsten wrote:

hello list,

i try to use r.shaded.relief on srtm tiles imported with r.in.srtm.
the results look nice, but adjacent tiles have a different average brightness.
I used the same parameters (altitude=44 azimuth=270 units=meters) for both maps and tried with grass versions 6.02 and 6.20
That's how it looks like for the original srtm tiles N42W005.hgt.zip
and N42W004.hgt.zip:
http://www.flickr.com/photos/h2guide/328146671/

What am i doing wrong? (Or what am i supposed to do in order to produce seamless shaded reliefs?)
   
try setting an absolute color table with r.colors,

or merge the two tiles first:

g.region rast=tile1,tile2
r.patch in=tile1,tile2 out=tile_merge

Hamish

4

thorsten wrote:

thank you Hamish,

patching works fine ( http://www.flickr.com/photos/h2guide/329431459 ),
but i try to create a shaded relief of a hughe area,
so i will have to process it tile by tile.

what i am actually doing is:

r.in.srtm input=N42W005 output=tile r.in.srtm input=N42W004 output=tile
g.region rast=tile
r.colors map=tile rules=srtm
r.shaded.relief map=tile shadedmap=relief altitude=44 azimuth=270
units=meters
r.his h_map=tile i_map=relief r_map=final.red g_map=final.green
b_map=final.blue
r.composite red=final.red green=final.green blue=final.blue levels=32
output=final;

"final" is the shaded and colored relief.

what do you mean by "absolute" color table?
i am using the predefined color table "srtm"

That's OK. The problem is that r.shaded.relief script internally uses
"r.colors "$ELEVSHADE" color=grey" internally. Since the "grey"
colortable is defined as follows:

$ cat /usr/local/grass-6.3.cvs/etc/colors/grey
0% black
100% white

the colortable depends on min/max value in the input raster.

A workaround: you can check the max and min height in *all* your
rasters, choose the maximal max an minimal min out of them, and modify
the r.shaded.relief script to assign your absolute-grey colortable instead.

Maciek

5

Thanks a lot!!
now i got it.
and Merry Xmas!

Maciej Sieczka wrote:

thorsten wrote:

thank you Hamish,

patching works fine ( http://www.flickr.com/photos/h2guide/329431459 ),
but i try to create a shaded relief of a hughe area,
so i will have to process it tile by tile.

what i am actually doing is:

r.in.srtm input=N42W005 output=tile r.in.srtm input=N42W004 output=tile
g.region rast=tile
r.colors map=tile rules=srtm
r.shaded.relief map=tile shadedmap=relief altitude=44 azimuth=270
units=meters
r.his h_map=tile i_map=relief r_map=final.red g_map=final.green
b_map=final.blue
r.composite red=final.red green=final.green blue=final.blue levels=32
output=final;

"final" is the shaded and colored relief.

what do you mean by "absolute" color table?
i am using the predefined color table "srtm"
   
That's OK. The problem is that r.shaded.relief script internally uses
"r.colors "$ELEVSHADE" color=grey" internally. Since the "grey"
colortable is defined as follows:

$ cat /usr/local/grass-6.3.cvs/etc/colors/grey
0% black
100% white

the colortable depends on min/max value in the input raster.

A workaround: you can check the max and min height in *all* your
rasters, choose the maximal max an minimal min out of them, and modify
the r.shaded.relief script to assign your absolute-grey colortable instead.

Maciek

6

I tried to update my grass source from cvs just now and was told that the
connection to intevation.de timed out. Is the Grass cvs server down?

Jerry

7

On Fri, 29 Dec 2006 11:04:51 -0600,
"Jerry Nelson" <gnelson@uiuc.edu> wrote:

I tried to update my grass source from cvs just now and was told that
the connection to intevation.de timed out. Is the Grass cvs server down?

I'm having the same problem here, so it might be.

--
Seb

8

Hello *,

On Fri, 29 Dec 2006 11:15:03 -0600 Seb <spluque@gmail.com> wrote:

On Fri, 29 Dec 2006 11:04:51 -0600,
"Jerry Nelson" <gnelson@uiuc.edu> wrote:

> I tried to update my grass source from cvs just now and was told
> that the connection to intevation.de timed out. Is the Grass cvs
> server down?

I'm having the same problem here, so it might be.

GRASS CVS server was down due to Telekom provider problems on Friday
about 17:00 CET. Currently the system is working, hopefully
reliable again as before!

Sorry for any inconvenience.

Best
  Stephan

--
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