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

[Paul Kelly]

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