Re: [GRASS-user] Get values from raster map following a direction map for n steps
On 23/11/18 14:28, Markus Metz wrote:
On Tue, Nov 20, 2018 at 9:16 AM Stefan Blumentrath
mailto:stefan.blumentr...@nina.no>> wrote:
>
> Dear all,
>
>
>
> In an r.mapcalc expression, I would like to trace values along a
direction map for 1 to n steps (e.g. looking 4 steps ahead along a path
or stream or along a given direction (degrees or 45degrees).
>
>
>
> I tried using the neighborhood modifier within an eval function in
r.mapcalc. Unfortunately, using this command:
>
> r.mapcalc --o << EOF
>
> eval(elev_200 = elevation[1,1] - 200, elev_5 = 5 * elev_200[1,1],
elev_p = pow(elev_5, 2))
>
> gradient_1 = (0.5 * elev_200) + 0.8 * elev_p
>
> EOF
elev_200 is an internal variable of the elev function with a single
value, and not a (temporary) map. Therefore elev_200[1,1] can't work.
You would need several calls to r.mapcalc, with the first one creating
the map elev_200, e.g.
r.mapcalc "elev_200 = elevation[1,1] - 200"
r.mapcalc "elev_p = pow(5 * elev_200[1,1], 2)"
r.mapcalc "gradient_1 = (0.5 * elev_200) + 0.8 * elev_p"
Cf the man page (Notes section):
"Any maps generated by a r.mapcalc command only exist after the entire
command has completed. All maps are generated concurrently, row-by-row
(i.e. there is an implicit "for row in rows {...}" around the entire
expression). Thus the #, @, and [ ] operators cannot be used on a map
generated within same r.mapcalc command run. "
Moritz
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Re: [GRASS-user] Get values from raster map following a direction map for n steps
On Tue, Nov 20, 2018 at 9:16 AM Stefan Blumentrath < stefan.blumentr...@nina.no> wrote: > > Dear all, > > > > In an r.mapcalc expression, I would like to trace values along a direction map for 1 to n steps (e.g. looking 4 steps ahead along a path or stream or along a given direction (degrees or 45degrees). > > > > I tried using the neighborhood modifier within an eval function in r.mapcalc. Unfortunately, using this command: > > r.mapcalc --o << EOF > > eval(elev_200 = elevation[1,1] - 200, elev_5 = 5 * elev_200[1,1], elev_p = pow(elev_5, 2)) > > gradient_1 = (0.5 * elev_200) + 0.8 * elev_p > > EOF elev_200 is an internal variable of the elev function with a single value, and not a (temporary) map. Therefore elev_200[1,1] can't work. You would need several calls to r.mapcalc, with the first one creating the map elev_200, e.g. r.mapcalc "elev_200 = elevation[1,1] - 200" r.mapcalc "elev_p = pow(5 * elev_200[1,1], 2)" r.mapcalc "gradient_1 = (0.5 * elev_200) + 0.8 * elev_p" Markus M > > > > I get the following error message: > > syntax error, unexpected '[', expecting ')' > > > > However, this command works fine: > > r.mapcalc --o << EOF > > eval(elev_200 = elevation[1,1] - 200, elev_5 = 5 * elevation[1,1], elev_p = pow(elev_5, 2)) > > gradient_1 = (0.5 * elev_200) + 0.8 * elev_p > > EOF > > > > My questions now are: > > a) is it generally not supported to use the neighbourhood modifier on a temporary map defined in an eval() function in r.mapcalc? > > b) are there other (efficient) approaches for those of us who do not know C (e.g. pygrass)? > > > > I am grateful for any hint. > > > > Kind regards, > > Stefan > > ___ > grass-user mailing list > grass-user@lists.osgeo.org > https://lists.osgeo.org/mailman/listinfo/grass-user ___ grass-user mailing list grass-user@lists.osgeo.org https://lists.osgeo.org/mailman/listinfo/grass-user
[GRASS-user] Get values from raster map following a direction map for n steps
Dear all, In an r.mapcalc expression, I would like to trace values along a direction map for 1 to n steps (e.g. looking 4 steps ahead along a path or stream or along a given direction (degrees or 45degrees). I tried using the neighborhood modifier within an eval function in r.mapcalc. Unfortunately, using this command: r.mapcalc --o << EOF eval(elev_200 = elevation[1,1] - 200, elev_5 = 5 * elev_200[1,1], elev_p = pow(elev_5, 2)) gradient_1 = (0.5 * elev_200) + 0.8 * elev_p EOF I get the following error message: syntax error, unexpected '[', expecting ')' However, this command works fine: r.mapcalc --o << EOF eval(elev_200 = elevation[1,1] - 200, elev_5 = 5 * elevation[1,1], elev_p = pow(elev_5, 2)) gradient_1 = (0.5 * elev_200) + 0.8 * elev_p EOF My questions now are: a) is it generally not supported to use the neighbourhood modifier on a temporary map defined in an eval() function in r.mapcalc? b) are there other (efficient) approaches for those of us who do not know C (e.g. pygrass)? I am grateful for any hint. Kind regards, Stefan ___ grass-user mailing list grass-user@lists.osgeo.org https://lists.osgeo.org/mailman/listinfo/grass-user
