[GRASS-user] How to rescale an aspect map

GRASS GRASS user
1

I need to rescale an aspect map in a scale 0-255 where North = 0 and SE=255.
Finally I need to assign weights as follow:
0.02 N
0.02 NE
0.12 E
0.34 SE
0.20 S
0.18 SW
0.08 W
0.03 NW

Thank you!

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2

No sure if I understood what you asked but here we go.

First I'd rescale (r.rescale) the aspect to the desired directions.
Take a look at the r.slope.aspect man page [1] to see what the aspect
values represent
Then I'd rescale the directions map to the weights you mention.

Hope it helps
Daniel

[1] - http://grass.osgeo.org/gdp/html_grass64/r.slope.aspect.html

On Wed, Feb 8, 2012 at 7:10 PM, leonidas <leonidas_liakos@yahoo.gr> wrote:

I need to rescale an aspect map in a scale 0-255 where North = 0 and SE=255.
Finally I need to assign weights as follow:
0.02 N
0.02 NE
0.12 E
0.34 SE
0.20 S
0.18 SW
0.08 W
0.03 NW

Thank you!

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3

I need the most suitable areas (with SE aspect) to have the value of 255 and
the unsuitable (North aspect) to have the value of 0. Grass returns N=90 and
SE=315 and flat=0.
So the minimum value of aspect raster is flat areas and the max is areas
with 359 degrees of east.
I should set North as the minimum value and SE as the max value (most
suitable areas).
I should rescale the raster because I need to include it in an AHP method.
All rasters will be rescaled in a common 0-255 scale.

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4

On 13/02/12 08:31, leonidas wrote:

I need the most suitable areas (with SE aspect) to have the value of 255 and
the unsuitable (North aspect) to have the value of 0. Grass returns N=90 and
SE=315 and flat=0.
So the minimum value of aspect raster is flat areas and the max is areas
with 359 degrees of east.
I should set North as the minimum value and SE as the max value (most
suitable areas).
I should rescale the raster because I need to include it in an AHP method.
All rasters will be rescaled in a common 0-255 scale.

Try r.recode with rules like this:

90:315:0:255
315:360:255:170
1:90:170:0
0:YourValueForFlatAreas

Moritz

5

Moritz,
how did you choose 170?

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6

On 13/02/12 11:16, leonidas wrote:

Moritz,
how did you choose 170?

315 = 255
90 = 0

Between the two you have 135 degrees (45 from 315 to 360 plus 90 from 1 to 90). 45 / 135 = 1/3, so 360 is at 255 - 255/3 = 170.

Obviously this means that you go from 255 to 0 much quicker when you move counter-clockwise than when you move clockwise. Up to you to decide whether this is what you want.

Moritz