Dear Edzer Pebesma,
thank your for your advice - you were right that the semivariance values are
standardized by default in the output of the nlme-Variogram function. So, with
the adequate "resType"-option in that function, I could reproduce nearly the
same estimations as in the gstat module:
> Variogram(lmm.dap2b, resType="response")
variog dist n.pairs
1 0.005385495 2.828427 122
2 0.007111336 4.269282 125
3 0.010111828 18.110770 123
4 0.009269399 20.000000 125
5 0.006815940 21.603952 126
....
The detailed information about the Variogram function, with all the possible
options, is found in the manual of the nlme package and not in the interactive
help, so at the moment of my first mail, I had not read these details about the
different modifiers.
Many thanks for your help,
Guido Lorenz
----- Original Message ----
From: Edzer Pebesma <edzer.pebe...@uni-muenster.de>
To: r-sig-geo@stat.math.ethz.ch
Sent: Tue, July 6, 2010 3:25:50 AM
Subject: Re: [R-sig-Geo] semivariogram calculation - discrepancy between gls
and gstat
Guido, I suspect that nlme works with models for spatial correlation and
not for spatial covariance, meaning that the variance (variog) gets
standardized - see ?corClasses in nlme. Do the variog values approach 1
for longer distances?
On 07/05/2010 11:02 PM, Guido Lorenz wrote:
> Dear R-sig-Geo members,
>
> a semivariogram for spatial soil data, calculated by the gls (nlme library)
> function gives the following estimates:
>
>> lmm.dap2b <- gls(Dap.sa ~ 1 + ID.sitio, Pd2006mm,
>> correlation=corGaus(form=~(easting.m+northing.m)|ID.sitio, nugget=TRUE,
>> metric="euclidean"), na.action=na.omit, method="REML")
>> Variogram(lmm.dap2b)
> variog dist n.pairs
> 1 0.3059585 2.828427 122
> 2 0.4040062 4.269282 125
> 3 0.5744688 18.110770 123
> 4 0.5266091 20.000000 125
> ....
>
> whereas the variogram function of the gstat library gives, for similar
> distances (although different number of sample pairs), very different gamma
> values:
>
> > variogram(Dap.sa ~ 1, locations = ~ easting.m + northing.m, data=Pd2006mm,
> cutoff=80)
> np dist gamma dir.hor dir.ver id
> 1 219 3.225693 0.005578128 0 0 var1
> 2 20 6.478671 0.004254656 0 0 var1
> 3 3 13.513045 0.009896324 0 0 var1
> 4 307 19.201322 0.009708390 0 0 var1
> .....
>
> Can anyone explain what is happening?
>
> Thanks for any advice,
> Guido Lorenz
>
> Can anyone explain
>
> _______________________________________________
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> R-sig-Geo@stat.math.ethz.ch
> https://stat.ethz.ch/mailman/listinfo/r-sig-geo
--
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.pebe...@wwu.de
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