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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