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

[Robert Hijmans]

Why not use cross-validation to empirically determine which method
performs best for this dataset (in addition to asking if they are
better than a random draw)?  Robert

2009/2/9 Tomislav Hengl <T.Hengl at uva.nl>:
>
> Dear Yong Li,
>
> I hope you will not mind me joining this interesting discussion.
>
> If there is no evident spatial auto-correlation structure (pure nugget effect), IDW/OK are as good
> as randomly drawing a value from the global (normal) distribution. You can even test this using
> cross-validation! In principle, there is no justification to use distance-based interpolators if
> there is no evident spatial auto-correlation structure (maybe only the moving-window kriging, as
> implemented in e.g. Vesper, or stratified kriging techniques could discover some local spatial
> dependence). In addition, IDW should be considered an outdated technique, applicable only for
> situations where the variogram is close to linear (e.g. elevation data and similar smooth surfaces).
>
> What you should really consider using are the globaly available free maps/images (e.g. MODIS EVI,
> SRTM DEM parameters etc.), and then see if you can explain some of the variability in your target
> variable.
>
> But there will always be situations (especially in DSM applications) where you simply can not
> explain much of the target variability, neither with auxiliary maps nor with spatial
> auto-correlation. What to do then? I guess you simply have to collect more samples / more auxiliary
> maps and then try again.
>
> HTH
>
> T. Hengl
>
> See also:
>
> Compendium of Global datasets:
> http://spatial-analyst.net/wiki/index.php?title=Global_datasets
>
> Regression-kriging:
> http://spatial-analyst.net/wiki/index.php?title=Regression-kriging
>
> Pebesma, E., 2006. The Role of External Variables and GIS Databases in Geostatistical Analysis.
> Transactions in GIS, 10(4): 615-632.
> http://dx.doi.org/10.1111/j.1467-9671.2006.01015.x
>
>
>> -----Original Message-----
>> From: r-sig-geo-bounces at stat.math.ethz.ch [mailto:r-sig-geo-bounces at stat.math.ethz.ch] On Behalf
>> Of Edzer Pebesma
>> Sent: Monday, February 09, 2009 9:08 AM
>> To: Yong Li
>> Cc: r-sig-geo at stat.math.ethz.ch
>> Subject: Re: [R-sig-Geo] FW: Interpolcation option: IDW or OK?
>>
>> 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
>>
>> _______________________________________________
>> R-sig-Geo mailing list
>> R-sig-Geo at stat.math.ethz.ch
>> https://stat.ethz.ch/mailman/listinfo/r-sig-geo
>
> _______________________________________________
> R-sig-Geo mailing list
> R-sig-Geo at stat.math.ethz.ch
> https://stat.ethz.ch/mailman/listinfo/r-sig-geo
>