r.drain

NAME

r.drain - Traces a flow through an elevation model on a raster map layer.
(GRASS Raster Program)

SYNOPSIS

r.drain
r.drain help
r.drain input=name output=name [coordinate=x,y[,x,y,...]]

DESCRIPTION

r.drain traces a flow through a least-cost path in an elevation model. The elevation surface (a raster map layer input) might be the cumulative cost map generated by the r.cost program. The output result (also a raster map layer) will show one or more least-cost paths between each user-provided location(s) and the low spot (low category values) in the input model.

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

r.drain input=name output=name [coordinate=x,y[,x,y,...]]

where the input name is the name of a raster map layer to be used in calculating drainage, the output name is the name of the raster map layer to contain output, and each x,y coordinate pair is the geographic location of a point from which drainage is to be calculated.

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.

OPTIONS

Program parameters are as follows.

Parameters:

input=name: Name of raster map layer containing cell cost information.
output=name: Name of raster map layer to contain program output.
coordinate=x,y[,x,y,...]: Each x,y pair is the easting and northing (respectively) of a starting point from which a least-cost corridor will be developed. As many points as desired can be input. (But, see BUGS below.)

EXAMPLE

Consider the following example:

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.

BUGS

Currently, r.drain will not actually provide output for more than one pair of input coordinates stated on the command line.

r.drain also currently finds only the lowest point (the cell having the smallest category value) in the input file that can be reached through directly adjacent cells that are less than or equal in value to the cell reached immediately prior to it; therefore, it will not necessarily reach the lowest point in the input file. It currently finds pits in the data, rather than the lowest point present.

Only one least-cost path is currently printed to the output file for the user.

Sometimes, when 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.

AUTHOR

Kewan Q. Khawaja, Intelligent Engineering Systems Laboratory, M.I.T.