With the -c (copy) flag, the input map cells values are copied verbatim along the path. With the -a (accumulate) flag, the accumulated cells value from the starting point up to the current cell is written on output. With the -c and the -a flags, the output map is created withe the same cell type than the input map (integer, float or double). The -c and -a flags are mutually incompatible. If both are specified, the -a flag will superceed the -c flag, and a warning message will be issued.
The path is calculated by choosing the stepper "slope" beetwen adjacent cells. The slope calculation accurately acounts for the variable scale in lat-lon projections.
The program will be run non-interactively, if the user specifies the names of raster map layers and any desired options on the command line, using the form
Alternately, the user can simply type r.drain on the command line, without program arguments. In this case, the user will be prompted for needed parameter values using the standard GRASS parser interface.
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. 28. 44. . . . . . |___| . . . . . . . . . . . . . . . . . . . . . . . . . . 18. 18. 24. 18. 15. 12. 11. . . . . . 59 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22. 16. 16. 18. 10. 10. 10. . . . . . 69 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17. 15. 15. 15. 10. 8 . 8 . . . . . . . 77 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24. 16. 8 . 7 . 8 . 0 .12 . . . . . . . 77 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17. 9 . 8 . 7 . 8 . 6 .12 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Sometimes, when using an integer cell map and the differences among 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 (r.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 input cost surface map to r.cost by the value of the map resolution, before running r.cost. This can be done using r.mapcalc or other programs. The map resolution can be found using g.region.
Update to Grass 5.0 format: Pierre de Mouveaux (pmx@audiovu.com)