Hello Gregoire, and list, I'd guess that, although you may be seeing some spatial structure in your data, the effective range of your model may be less than the minimum distance between the locations you are predicting and their closest known locations. In other words, your model may effectively be a 'pure nugget', even though technically it is not.
One check on how much kriging is improving predictions is to see how much better cross validated kriging predictions do than a simple unweighted mean for each location. >From memory (and therefore very unreliable!) I seem to recall that radon is generally very spatially noisy across relatively small geographic areas, so spatial interpolation may simply not be a very effective technique, at least without additional variables. Yours, Ashton > > I thought about that (the nugget effect) but my variogram (and robust > > variograms) did show a short scale structure (with a high nugget > > effect). > > > > Considering the possibility you mention, that I actually have a > > phenomenon with a pure nugget effect although I see a structure, what do > > I have to trust? My experimental variogram showing a structure or the > > strong slope of my residuals (either + or -1 depending on the way you > > define the residuals) plotted against the observed data telling me that > > I actually may not have a structure? The correlation between my observed > > and estimate values is poor indeed (0.40) but still.. > > > > For what concerns the definition of the residuals, Surfer and Isatis > > used here defined the residuals = estimate - observed. I haven't > > checked the other software. > > > > I had a quick look in Isaaks & Srivastava, Deutsch & Journel, Chiles and > > Delfiner... all use the same definition as above (i.e. residuals = > > estimate - observed). > > > > Many thanks again for your kind feedback! > > > > Gregoire > > > > __________________________________________ > > Gregoire Dubois (Ph.D.) > > > > European Commission (EC) > > Joint Research Centre (DG JRC) > > WWW: http://rem.jrc.cec.eu.int/ > > > > "The views expressed are purely those of the writer and may not in any > > circumstances be regarded as stating an official position of the > > European Commission." > > > > -- Ashton Shortridge Associate Professor [EMAIL PROTECTED] Dept of Geography http://www.msu.edu/~ashton 235 Geography Building ph (517) 432-3561 Michigan State University fx (517) 432-1671 + + To post a message to the list, send it to [email protected] + To unsubscribe, send email to majordomo@ jrc.it with no subject and "unsubscribe ai-geostats" in the message body. DO NOT SEND Subscribe/Unsubscribe requests to the list + As a general service to list users, please remember to post a summary of any useful responses to your questions. + Support to the forum can be found at http://www.ai-geostats.org/
