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I have an elevation raster map, a point, a raster map defining the viewshed from that point, and now I want to produce another raster map with the results of a function of the distance between the point and every point (cell) in the elevation map that has a defined value (not equal to 180) in the viewshed raster map. The function the distance that I want to compute between the point and the cells in the elevation map is the inverse square law.

How do I do that using GRASS?

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    You need to state which GIS and which version you are using.
    – Hornbydd
    Sep 13, 2015 at 15:16
  • GRASS 6.4.1 (2011) running on Ubuntu 12.04 LTS
    – apoyoman
    Sep 13, 2015 at 15:31

1 Answer 1

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You could use the r.buffer function. But it uses classes for the distance.

r.buffer input=roads output=roads.buf distances=100,200,300,400,500

The result would be:

  roads                    roads.buf
  000000000000000000000000 222233333333333333444444 
  111000000000000000000000 111222222222223333333333 
  000111111111100000000000 222111111111122223333333 
  000000001000011100000000 332222221222211122222222 
  000000001000000011111111 333333321233222211111111 
  000000001000000000000000 433333321233333222222222 
  000000001000000000000000 444443321233333333333333 
  000000001000000000000000 444443321233443333333333 
  000000001000000000000000 444443321233444444444444 
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    Better use r.grow.distance (grass.osgeo.org/grass70/manuals/r.grow.distance.html) which generates a raster map containing distances to nearest raster features.
    – markusN
    Sep 18, 2015 at 10:20
  • Wow. Ideally would want the actual 3-dimensional distance taking into account all of the elevations in the elevation raster map corresponding only to those points that are non-null in the viewshed raster. The point also has its own height which may be independent of the height of the topography in the elevation raster.
    – apoyoman
    Oct 8, 2015 at 17:22
  • 3D distance wouldn't be much more difficult. r.grow.distance can give you r^2 = x^2+y^2. You could use r.mapcalc to get z^2, e.g., r.mapcalc z2=(z_elevation-z_point)^2. And use r.mapcalc again, e.g., r.mapcalc 3d_dist=sqrt( r2 + z2 ).
    – user55937
    Feb 24, 2016 at 13:43

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