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.<br>My dataset is composed of 122 points, and the attribute to be interpolated is Z, in the sample I attach.<br>
I've interploated first in a narrow region around a peak. 5/122 points were used and the result is this:<a href="http://www.geospatial.it/allegri/IDW1.png">http://www.geospatial.it/allegri/IDW1.png</a>, 491 rows x 552 columns of 2 meters cells. <br>
Then I've widened the region in the same area: <a href="http://www.geospatial.it/allegri/IDW2.png">http://www.geospatial.it/allegri/IDW2.png</a>. Ok<br>Then, when I've interpolated across the while area, the result is this strange over-smoothed surface: <a href="http://www.geospatial.it/allegri/IDW3.png">http://www.geospatial.it/allegri/IDW3.png</a> , where I put in evidence the first small region.<br>
Another wierd surface I got is: <a href="http://www.geospatial.it/allegri/IDW4.png">http://www.geospatial.it/allegri/IDW4.png</a> , resulted from a lower resolution setting (50 meters) of the region.<br><br>About IDW3:<br>
Why the interpolation fails to detect local anomalies while it gets wider?<br>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...<br>
<br>I attach the sample shapefile with a "Z" attribute representing my interpolation values: <a href="http://www.geospatial.it/allegri/sample_allegri.zip">http://www.geospatial.it/allegri/sample_allegri.zip</a><br>
<br>Giovanni<br>