On Sunday 21 June 2009, Ebrahim Jahanshiri wrote: > It has been long that I wanted to suggets this for automatic trend > detection based on our previous conversations with Edzer and Anne. I > found two ways that seem to be reasonable and have potential for > automizing the trend detection ( I got these from my conversations > with Margaret Oliver and Dick Bruc both prominent soil scientists): > > 1-If we fit some 2D surfaces and look at the percentage variance > accounted for by the surface . If it is much more than 25% then you > have trend that needs to be dealt with for geostatistics. > > [By "fitting a surface" I think means we can use the default > parameters for just creating a surface. i dont know if we could use > other methods lik IDW for just checking] > > this can be done pretty well in ArcGIS but I am sure that we could > come up with something to do it automatically and give the user the > result... > > 2- For detecting the order, If we fit the trend by Ordinary Least > Squares (which implies that we assume the residuals are independent) > we can test whether the regression coefficients differ significantly > from 0. > > linear trend: > > model z > fit x, y > > > second order polynomial: > > model z > fit x, y, xy, x2, y2 > > or any other order.
Good ideas and nice to see prominent soil scientists contributing to the conversation! As for an approach towards automatic trend detection: we may be able to skip parametric specification of the trend (i.e. poly(2,3,4,5, ...)) by using some kind of smoother: restricted cubic splines comes to mind (see rcs() in the Design package). The script could then check the various elements of an RCS fit for a pre-defined amount of variance explained in order to suggest trend-removal. Cheers, Dylan -- Dylan Beaudette Soil Resource Laboratory http://casoilresource.lawr.ucdavis.edu/ University of California at Davis 530.754.7341 _______________________________________________ R-sig-Geo mailing list R-sig-Geo@stat.math.ethz.ch https://stat.ethz.ch/mailman/listinfo/r-sig-geo