[R-sig-Geo] FW: Interpolcation option: IDW or OK?

Edzer Pebesma edzer.pebesma at uni-muenster.de
Mon Feb 9 09:08:14 CET 2009

Yong Li wrote:
> Hi Edzer,
> I would say the spatial structure is regarded not significant when c0/c0+c1 is very much greater than 75%. In my case I used even distance intervals and calculated c0/c0+c1 for log(OLSENP) greater than 85%. I knew this index sometimes is very fragile, very much depending on how we fit the model.
> However when I zoomed in by using variable distance intervals (boundaries=c(100,200,300,400,600,900,1000,1500,2000))and maxdist=2000 meters, I found a pretty good model-fitted experimental variogram. But the local OK interpolation using such a fitted model did not make sense when compared the predictions to the observations as in most areas values of OLSENP were severely underestimated. You may have seen my code with which I have tried the nested models, but unfortunately no luck either. I maybe think the parameters for local ordinary kriging are not optimized, but I have tried lots of sets of nmin, nmax and maxdist and did see the hopeful end.
> The journal editor insists in OK being better than IDW. I need to collect my evidence to defend my IDW choice. That is my intention raised such a question in our forum here.
I cannot find evidence in your data for such a claim; the cross 
validation statistics (rmse) seem to favour OK with your nested model.

In your first email, you stated the following:
>> Normally if we do not find a significant spatial structure for a soil
>> variable, we may choose IDW or other methods. 
What is the argumentation behind this? Who claimed this?

Edzer Pebesma
Institute for Geoinformatics (ifgi), University of Münster
Weseler Straße 253, 48151 Münster, Germany. Phone: +49 251
8333081, Fax: +49 251 8339763 http://ifgi.uni-muenster.de/
http://www.springer.com/978-0-387-78170-9 e.pebesma at wwu.de

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