Hi Jin,
[EMAIL PROTECTED] wrote:
Hi Edzer,
It is true that demo(cokriging) shows that it is using ordinary cokriging,
but if we examine its formula, it kriges multiple primary variables without
secondary information. The ordinary co-kriging (OCK) I mentioned (such as
those defined in: Goovaerts, 1997. Geostatistics for Natural Resources
Evaluation.) interpolates one primary variable using one or more secondary
variables to improve the estimations. I guess that is the key difference.
Ordinary cokriging and multivariate kriging (as e.g. described in the
original papers by Don Myers in Math Geol, or in Noel Cressie's book, or
Ver Hoef and Cressie MG) are mathematically equivalent. If you would
constrain multivariate kriging to kriging of the first variable only
(the Goovaerts formulation), you end up with exactly the same values
(and gain a little CPU wise).
In demo(examples), the multivariate kriging as shown in ex11.cmd claims it is
using ordinary cokriging, but it actually has no difference with that in
demo(cokriging), i.e. it kriges multiple primary variables without secondary
information.
It uses the "secondary" information for the first because a cross
variogram is defined.
I thought multiple kriging in ex10.cmd in demo(examples) was the one I was
after. After a further check, it is the ordinary kriging. I hope this
explanation is clear enough.
No, this "multiple" kriging kriges each variable completely
independenlty, as no cross variogram is defined. It was introduced in
gstat stand-alone to speed up multiple kriging settings (think of
several indicators derived from the same variable) as it re-uses the
neighbourhood selection. I wasn't aware of it, but it seems to work in
the R package as well (set up a cokriging gstat object, but don't define
cross variograms).
Thanks a lot for any further suggestions.
Best wishes,
Jin
-----Original Message-----
From: Edzer Pebesma [mailto:[EMAIL PROTECTED]
Sent: Monday, 18 August 2008 6:32
To: Li Jin
Cc: [email protected]
Subject: Re: [R-sig-Geo] UK, KED or OCK? [SEC=UNCLASSIFIED]
Hi Jin,
Which differences do you exactly refer to? I'd say that what happens in
demo(cokriging) is ordinary cokriging. As it says:
...
[using ordinary cokriging]
Best regards,
--
Edzer
[EMAIL PROTECTED] wrote:
Hi All,
I am going to compare a few spatial interpolation techniques including
kriging with an external drift (KED) and ordinary co-kriging (OCK) (such as
those defined in: Goovaerts, 1997. Geostatistics for Natural Resources
Evaluation.) to interpolate marine sediment data (mud content in this case)
using bathymetry as a secondary variable. However, it seems that the
ordinary
cokriging in gstat as shown in demo(cokriging) is different from the OCK we
planned to use. Is it possible to do such OCK in gstat? Any comments and
example? Thanks.
As to KED, I tried
vgm1 <- variogram(sqrt(mud)~bathy, data.file.dev)
model.1 <- fit.variogram(vgm1,vgm(1,"Sph",5,1))
# plot(vgm1, model.1)
coordinates(data.file.pred) = ~LON+LAT
mud.ok <- krige(sqrt(mud)~bathy, data.file.dev, data.file.pred, model =
model.1)
[using universal kriging]
vgm1 <- variogram(sqrt(mud)~LON+LAT, data.file.dev)
model.1 <- fit.variogram(vgm1,vgm(1,"Sph",5,1))
# plot(vgm1, model.1)
coordinates(data.file.pred) = ~LON+LAT
mud.ok <- krige(sqrt(mud)~LON+LAT, data.file.dev, data.file.pred, model =
model.1)
[using universal kriging]
Both of them are UK. But the first one seems regression kriging. Is it
identical to KED in this case? If not, any comments and examples of KED are
appreciated.
Cheers,
Jin
--------------------------------------------
Jin Li, PhD
Spatial Modeller/
Computational Statistician
Marine & Coastal Environment
Geoscience Australia
Ph: 61 (02) 6249 9899
Fax: 61 (02) 6249 9956
email: [EMAIL PROTECTED] <mailto:[EMAIL PROTECTED]>
--------------------------------------------
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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/
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