[GRASS-user] strange behaviour of v.surf.idw

[G. Allegri]

Hello everyone. In these days I was using IDW to make an easy interpolation
of a point features layer, and I faced a strange behaviour.
My dataset is composed of 122 points, and the attribute to be interpolated
is Z, in the sample I attach.
I've interploated first in a narrow region around a peak. 5/122 points were
used and the result is this:http://www.geospatial.it/allegri/IDW1.png, 491
rows x 552 columns of 2 meters cells.
Then I've widened the region in the same area:
http://www.geospatial.it/allegri/IDW2.png. Ok
Then, when I've interpolated across the while area, the result is this
strange over-smoothed surface: http://www.geospatial.it/allegri/IDW3.png ,
where I put in evidence the first small region.
Another wierd surface I got is: http://www.geospatial.it/allegri/IDW4.png ,
resulted from a lower resolution setting (50 meters) of the region.

About IDW3:
Why the interpolation fails to detect local anomalies while it gets wider?
It seems that the algorithm doesn't manage correctly the incresing number of
points vs search radius. I will try to take a look at the v.surv.idw code,
and to understand what the nrowsxncols/npoints>400 threshold stands for...

I attach the sample shapefile with a "Z" attribute representing my
interpolation values: http://www.geospatial.it/allegri/sample_allegri.zip

Giovanni
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