[GRASS-user] Re: Defining Partial Drainage Basin

[Micha Silver]

On 02/11/2010 12:23 AM, Rich Shepard wrote:
> 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.
>
How about:
 > v.db.addcol house_basin col="area_sqm DOUBLE PRECISION"
 >v.to.db house_basin opt=area col=area_sqm units=meters

>   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
>
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-- 
Micha Silver
http://www.surfaces.co.il/
Arava Development Co.  +972-52-3665918