[GRASS-user] r.cost source ID question...

[Dylan Beaudette]

On Friday 29 February 2008, Jonathan Greenberg wrote:
> If I have a r.cost surface calculated from a set of vector points used
> as starting points, and some cost surface, is there any way to easily
> retrieve the SOURCE vector ID that leads to the lowest cost to get to
> any cell in the raster?  E.g. from a given point, where is the point
> that costs the least to get to?
>
> --j

r.cost is generally used in conjunction with r.drain.

First a cost surface from:
- a starting point
- a set of starting points
- a line segment

is generated to:
- the edges of the region
- a stopping point
- a set of stopping points

Then r.drain is used to compute the least-cost-path between the starting 
point/points/line and some arbitrary end point. With the idea that you 
will "drain" some water down-slope the cost surface, back to your starting 
point/points/line.

To find the least-cost-path between *any two points* I have generally found it 
simplest to run r.cost + r.drain for every start/stop combination that I am 
interested in. An example is posted here:

http://casoilresource.lawr.ucdavis.edu/drupal/node/244

note that the code describing this process on the referenced page is actually 
an un-rolled foreach loop.
 
Hope that helps a little,

Dylan


-- 
Dylan Beaudette
Soil Resource Laboratory
http://casoilresource.lawr.ucdavis.edu/
University of California at Davis
530.754.7341