With the -c (copy) flag, the input map cell values are copied verbatim along the path. With the -a (accumulate) flag, the accumulated cell value from the starting point up to the current cell is written on output. With either the -c or the -a flags, the output map is created with the same cell type as the input map (integer, float or double). With the -n (number) flag, the cells are numbered consecutively from the starting point to the final point. The -c, -a, and -n flags are mutually incompatible.
For an elevation surface, the path is calculated by choosing the steeper "slope" between adjacent cells. The slope calculation accurately acounts for the variable scale in lat-lon projections. For a cost surface, the path is calculated by following the movement direction surface back to the start point given in r.walk or r.cost.
The coordinate parameter consists of map E and N grid coordinates of a starting point. Each x,y pair is the easting and northing (respectively) of a starting point from which a least-cost corridor will be developed. The vector_points parameter can take multiple vector maps containing additional starting points. Up to 1024 starting points can be input from a combination of the coordinate and vector_points parameters.
r.drain will not give sane results at the region boundary. On outer rows and columns bordering the edge of the region, the flow direction is always directly out of the map. In this case, the user could try adjusting the region extents slightly with g.region to allow additional outlet paths for r.drain.
Input: Output: ELEVATION SURFACE LEAST COST PATH . . ----- . . . . . . . . . . . . . . . . . . . . . . . . . . 20| 19| 17. 16. 17. 16. 16. . . 1 . 1 . 1 . . . . . . |___| . . . . . . . . . . . . . . . . . . . . . . . . . . 18. 18. 24. 18. 15. 12. 11. . . . . . 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22. 16. 16. 18. 10. 10. 10. . . . . . 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17. 15. 15. 15. 10. 8 . 8 . . . . . . . 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24. 16. 8 . 7 . 8 . 0 .12 . . . . . . . 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17. 9 . 8 . 7 . 8 . 6 .12 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
The user-provided starting location in the above example is the boxed 19 in the left-hand map. The path in the output shows the least-cost corridor for moving from the starting box to the lowest (smallest) possible point. This is the path a raindrop would take in this landscape.
With the -c (copy) flag, you get the following result:
Input: Output: ELEVATION SURFACE LEAST COST PATH . . ----- . . . . . . . . . . . . . . . . . . . . . . . . . . 20| 19| 17. 16. 17. 16. 16. . . 19. 17. 16. . . . . . |___| . . . . . . . . . . . . . . . . . . . . . . . . . . 18. 18. 24. 18. 15. 12. 11. . . . . . 15. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22. 16. 16. 18. 10. 10. 10. . . . . . 10. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17. 15. 15. 15. 10. 8 . 8 . . . . . . . 8 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24. 16. 8 . 7 . 8 . 0 .12 . . . . . . . 0 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17. 9 . 8 . 7 . 8 . 6 .12 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Note that the last 0 will not be put in the null values map.
With the -a (accumulate) flag, you get the following result:
Input: Output: ELEVATION SURFACE LEAST COST PATH . . ----- . . . . . . . . . . . . . . . . . . . . . . . . . . 20| 19| 17. 16. 17. 16. 16. . . 19. 36. 52. . . . . . |___| . . . . . . . . . . . . . . . . . . . . . . . . . . 18. 18. 24. 18. 15. 12. 11. . . . . . 67. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22. 16. 16. 18. 10. 10. 10. . . . . . 77. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17. 15. 15. 15. 10. 8 . 8 . . . . . . . 85. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24. 16. 8 . 7 . 8 . 0 .12 . . . . . . . 85. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17. 9 . 8 . 7 . 8 . 6 .12 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
With the -n (number) flag, you get the following result:
Input: Output: ELEVATION SURFACE LEAST COST PATH . . ----- . . . . . . . . . . . . . . . . . . . . . . . . . . 20| 19| 17. 16. 17. 16. 16. . . 1 . 2 . 3 . . . . . . |___| . . . . . . . . . . . . . . . . . . . . . . . . . . 18. 18. 24. 18. 15. 12. 11. . . . . . 4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22. 16. 16. 18. 10. 10. 10. . . . . . 5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17. 15. 15. 15. 10. 8 . 8 . . . . . . . 6 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24. 16. 8 . 7 . 8 . 0 .12 . . . . . . . 7 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17. 9 . 8 . 7 . 8 . 6 .12 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
The directions are recorded as GRASS standard directions: 112.5 67.5 i.e. a cell with the value 135 157.5 135 90 45 22.5 means the cell before it is 180 x 0 to the south-east. 202.5 225 270 315 337.5 247.5 292.5
Sometimes, when the differences among integer cell category values in the r.cost cumulative cost surface output are small, this cumulative cost output cannot accurately be used as input to r.drain (t.drain will output bad results). This problem can be circumvented by making the differences between cell category values in the cumulative cost output bigger. It is recommended that if the output from r.cost is to be used as input to r.drain, the user multiply the r.cost input cost surface map by the value of the map's cell resolution, before running r.cost. This can be done using r.mapcalc. The map resolution can be found using g.region. This problem doesn't arise with floating point maps.
July 2004 at WebValley 2004, error checking and vector points added by Matteo Franchi (Liceo Leonardo Da Vinci, Trento) and Roberto Flor (ITC-irst, Trento, Italy)
Last changed: $Date: 2010-04-01 14:14:53 -0800 (Thu, 01 Apr 2010) $
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