[STATSGRASS] gstat: problem with prediction value at point location

[Roger Bivand]

On Tue, 23 Jan 2007, Edzer J. Pebesma wrote:

> My guess is that the technique you're referring to (and used by 
> Geostatistical Analyst, afaik) is [kriging in] cross 
> validation---observations are left out to obtain a kriging estimate for 
> which it makes sense to compare it with the observation. Definitely 
> distinct from kriging, which does not leave observations out.

When run head to head, gstat CV LOO (leave one out) on trend surfaces and 
GA give the same results, so, yes, GA seems by default to do LOO CV.

Roger

> --
> Edzer
> 
> Jaros³aw Jasiewicz wrote:
> > Edzer J. Pebesma napisa³(a):
> >> This is a well-known property of kriging if locations coincide 
> >> exactly: if measured, the best estimate is the measurement itself. 
> > Hmm. yes and no. As I remember form other program (like geostatistic 
> > analist in arc info) in point value wasn't exact. the difference 
> > between exact and predicted values in point location were used to 
> > calculate the RMS total (I can be wrong here, I didn't used Arc Info 
> > for over 2 years) before final calculation.
> >
> > I I attached samples, how it looks like with point location.
> >
> > It was found  during disscution when I presended some data. I was 
> > asked about RMS Total for my prediction (in arc info mode  - as sum of 
> > differences between exact and predicted values in point location). I 
> > tried to do this with sampling points at predicted map. In that way 
> > the RMS Total was 0!. I consult result with others and it seems 
> > variogram is OK,
> > (it was automatically fitted expotential vaiogram, for gaussian 
> > variogram result was worse).
> >> It's also cause of the problem that you can't have duplicate 
> >> observations in kriging. Even with a nugget, gamma(0)=0. You may want 
> >> to shift your points or your grid e.g. a few centimeters to get rid 
> >> of it.
> > it gives the same results
> > regards
> > Jarek
> >
> >
> > ------------------------------------------------------------------------
> >
> >
> > ------------------------------------------------------------------------
> >
> 
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-- 
Roger Bivand
Economic Geography Section, Department of Economics, Norwegian School of
Economics and Business Administration, Helleveien 30, N-5045 Bergen,
Norway. voice: +47 55 95 93 55; fax +47 55 95 95 43
e-mail: Roger.Bivand at nhh.no