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

GRASS GRASS user
1

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

2

On Tue, Nov 20, 2018 at 9:16 AM Stefan Blumentrath <Stefan.Blumentrath@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

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3

On 23/11/18 14:28, Markus Metz wrote:

On Tue, Nov 20, 2018 at 9:16 AM Stefan Blumentrath <Stefan.Blumentrath@nina.no <mailto:Stefan.Blumentrath@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