## Re: AI-GEOSTATS: gstat in R doubts

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• (I added gstat-info to the addressees, as I believe this message and possibly further discussion better belong there) -- Edzer ... If your variable is called
Message 1 of 2 , Nov 28, 2003
(I added gstat-info to the addressees, as I believe this message and
possibly further discussion better belong there)
--
Edzer

Marta Rufino wrote:

> Dear Collegues,
>
> I have some particular doubts about gstat for R (most probabily very
> basic for what I apoligise):
>
> 1. Does it compute indicator kriging? How to define it? It should be
> I=.5, but where? I could not find the argument in 'variogram'

If your variable is called z, and cutoff is .5, define the formula for
the data as:
I(z < .5) ~ 1

>
> 2. I could not manage to make it do the backtransformation of the
> kriging predictions when using log data. Is it possible and how?

You can take the exponent to get a median-like estimator, for
the expected value you will need to use the kriging variance as
well. See any book on lognormal kriging. The lagrange parameter
is not available.

>
>
> 3. How exactly we specify universal kriging? I have found different
> ways in different places:
>
> kk=krige(abu~x+y, ~x+y, dat, bl, model=wls) # I think this one
> indicates a quadractic internal trend :-)
> or
> kk=krige(abu~(covariate), ~x+y, dat, bl, model=wls)
> or
> kk=krige(abu~x+y+(covariate), ~x+y, dat, bl, model=wls)

These are indeed three forms of universal kriging (although some people
call the second "external drift" kriging -- but how then would they call
the third? Universal external drift kriging?).

BTW abu~x+y indicates a first order linear trend in x and y, use
abu~I(x^2)+I(y^2)+I(x*y)+x+y
for a second order linear trend (is that what you meant by quadratic trend?)

>
> And the variogram of the residuals for the universal kriging... how do
> we do it? the same way?

Yes: variogram(abu~x+y, ~x+y, dat)

>
> 4. The function plot.variogram as the same name as in geoR. This
> causes bugs and problems runing it, and does not allow us to run both
> packages which would be important. Could this be solved by any of the
> authors, simply by renaming the function... probabily there are more
> like this, although I did not proceed because I got stuck.
> (To solve this the only way I found is to attach and detach the
> packages, ...)

Paulo and I are aware of this; we'll solve it, at some stage.

> 5. The variogram produced by both packages (gstat and geoR) are
> different. Why?

Because Paulo Ribeiro and I are different persons. gstat is much older
than geoR,
but gstat in R is much younger. I copied the "old" gstat variogram
properties to the
gstat in R variogram object. I was in a hurry and didn't look too much
around. I can
regret this, but it's too late. I agree that they should have different
names. Give me
a good suggestion!

> It was excelent and extremely usefull for me the implementation of
> gstat in R!!! Thank you so much!

Thanks,
--
Edzer

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