[GRASS-user] Get values from raster map following a direction map for n steps

[Moritz Lennert]

On 23/11/18 14:28, Markus Metz wrote:
> 
> 
> On Tue, Nov 20, 2018 at 9:16 AM Stefan Blumentrath 
> <Stefan.Blumentrath at nina.no <mailto:Stefan.Blumentrath at 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