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

[G. Allegri]

Hi Paul. The anomalies are in the raster values, indipendently by the color
table. If you do an r.what around the peak in IDW1 you get raster values
corresponding to the point Z value (about 22000), while if you do r.what on
IDW3 you get values arounf 2000...

2008/2/12, Paul Kelly <paul-grass at stjohnspoint.co.uk>:
>
> On Tue, 12 Feb 2008, G. Allegri wrote:
>
> > 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.
>
> Are you sure they are really that different? If you could use the same
> colour table for all three images, it would be a lot easier to see. I
> mean, are the values actually very different or are you just going by the
> colours?
>
> If you still notice anomalies after assigning the same colour table to all
> the maps, I will definitely look into it, as I wrote a lot of the code in
> the current version of v.surf.idw.
>
> > 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'm not sure what you mean here. Can you explain further?
>
> > I will try to take a look at the v.surv.idw code,
> > and to understand what the nrowsxncols/npoints>400 threshold stands
> for...
>
> It means, if the resolution is quite a lot larger than the number of
> points, it simply searches through all the points for each cell of the
> output raster to find the 12 closest, rather than using a search radius to
> only search those close by. It is just a bit faster. But if you can
> improve the way it detects this it would be very interesting as I'm not
> happy with the choice of such an arbitrary number.
>
> Paul
>
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