[GRASS-dev] v.surf.idw broken

[Paul Kelly]

On Tue, 13 Apr 2010, Jachym Cepicky wrote:

> Hi,
> I can not get reasonable result (compared with v.surf.rst) from
> v.surf.idw using attached input vector points
>
> is it possible, that v.surf.idw is kind of broken?

Hello,
I can confirm that I can reproduce the strangeness, that it only happens 
when *not* using the -n flag (i.e. when indexing points) and that it 
happens when interpolating a relatively small number of points into a 
region with a relatively high resolution. E.g. for the given data I 
observe strangeness for a resolution of 10, but it goes away either by 
specifying the -n flag or decreasing the resolution to 15.

It does indeed look to be caused by my buggy code from 2003, or (perhaps 
less likely; I'm not sure) some later change to it - specifcally something 
to do with the "shortlist" method of avoiding an intensive search for 
nearby sites/points when the number of points is relatively small compared 
to the region size.

I can't see anything obviously wrong but I'll look into it over the next 
few days...

Paul