[GRASS-user] Re: Defining Partial Drainage Basin

[Rich Shepard]

On Wed, 10 Feb 2010, Micha Silver wrote:

> r.to.vect -s house_catchment out=house_catchment feature=area

Micha,

   This process worked flawlessly ... once I corrected slightly off
coordinates for the dam point. The last step I need is a way to calculate
the areas of these two partial basins. They have no attribute information
other than a category number.

   I tried v.overlay (intersecting the two maps) thinking that would
calculate the area of the smaller (b) map from that of the larger (a) map.
Doesn't work that way.

   My next try is to set the region to each of these partial basins,
one-by-one, then calculate the area by multiplying the number of cells by
100 (10x10m resolution) and converting that to acres. This does not seem
sufficiently precise to calculate the difference in drainage areas.

   r.stream.stats does not appear to help because that's working on the
raster maps and not the vector versions.

   Isn't there a way to calculate areas of the vectors produced by the
r.water.outlet-to-vector process?

Rich