(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
