[GRASS-user] Surface

[Paul Kelly]

On Wed, 10 Sep 2008, Raphael Saldanha wrote:

[...]
> I need to create countour... so, I think, I must interpolate a surface from
> this points making something like a DEM, and after, interpolate the
> contours... Is this correct?

Sounds like a good approach.

> Now, I'm trying to interpolate a suface with IDW (very slow...)
>
> v.surf.idw input=pontos2 at PERMANENT output=teste npoints=12 layer=1
> column=elevation --overwrite

You should use layer=0 if you want to read the elevation co-ordinates as 
the 3rd dimension of the vector data. The way you have written it is 
reading these from a database field which will slow it down a bit - you 
don't need a database connection as you are storing the elevation data as 
part of the vector map itself.

> Another question: what is the precision of this surface? What contour
> interval can I generate?

The elevation raster map will be generated according to the extents and 
resolution of your current region. g.region -p will show this. If your 
region is too big (i.e. extends beyond the region covered by the points) 
or the resolution is too high that could be another reason why it runs 
slowly. In general v.surf.idw should be quite fast.

Perhaps that gives you something to go on.

Paul