[R-sig-Geo] maxdist for kriging with an external drift
Edzer Pebesma
edzer.pebesma at uni-muenster.de
Tue Jan 26 21:11:47 CET 2010
Oh, geostatistics and its funny naming conventions!
I see local models vs. global models as a completely different modelling
aspect (model decision, basically) then the SK/OK/UK differences. When
building on the same tradition / body of literature you quote: in that
case KED would be a special form of UK, having only a single
non-coordinate predictor called 'external drift'.
In my eyes (and that of the literature with more mathematical
statistical grounding, such as Cressie 1993 and others), the difference
between SK on the one hand and OK/UK on the other is that SK assumes
that you know the mean or mean structure. SKlm is then residual kriging
added to a known mean function.
In the gstat R package you obtain SK by specifying a beta value (for the
mean); SKlm by specifying one or more predictors and passing the (known)
regression coefficients as beta; you obtain OK/UK by not specifying
beta; a formula ending on ~1 results in OK with an unknown mean only.
Ah, and then SK = simple kriging, OK = ordinary kriging, UK = universal
kriging.
--
Edzer
Cutberto Uriel Paredes Hernández wrote:
> Dear Edzer,
>
> Would it be correct to say then that if a neighbourhood is specified
> in the krige command the result would be that of Kriging with an
> External Drift (KED), otherwise it would be that of Simple Kriging
> with varying local means (SKlm)?
>
> Apologies for posting on this thread but I was about to post a
> similiar question.
>
> Thanks, Cutberto.
>
> 2010/1/26 Edzer Pebesma <edzer.pebesma at uni-muenster.de>:
>
>> Yes, that is right.
>>
>> Els Verfaillie wrote:
>>
>>> Dear list,
>>>
>>>
>>> I want to use Kriging with an external drift for a sedimentological
>>> dataset
>>> of grain-size that has a linear relation with the depth.
>>> Am I correct that when I set a 'maxdist' using the krige command, that a
>>> trend for the primary variable (grain-size) is calculated as a local
>>> linear
>>> function of the secondary variable (depth)? Is this function thus
>>> different
>>> for each interpolation window?
>>>
>>> d50.ked.dir50 <- krige(D50F~depth, locations=ds50, newdata=Depth,
>>> model=d50.fit.var.50, nmin=2, nmax=16, maxdist=9000)
>>>
>>>
>>> Thank you for your help.
>>>
>>>
>>> Best regards,
>>>
>>> Els Verfaillie
>>>
>>>
>>> ______________________________________________
>>>
>>> Dr. Els Verfaillie
>>>
>>> Carto-GIS cluster
>>>
>>> Ghent University (UGent) - Department of Geography
>>>
>>> ______________________________________________
>>>
>>>
>>>
>>>
>>> [[alternative HTML version deleted]]
>>>
>>> _______________________________________________
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>>>
>> --
>> 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.52north.org/geostatistics
>> e.pebesma at wwu.de
>>
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>>
--
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.52north.org/geostatistics e.pebesma at wwu.de
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