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variances

Hello,

I'm using universal kriging to create elevation maps of salt marshes.
After some trial and error variograming, I finally fitted some Gaussian
variograms (anisotropy included) through the data. When I looked at one
of the krigged maps, I found out that the variance map showed almost
only negative values. I don't believe that it is possible for the
variance to have a negative value! Can anyone explain this?

The script looks as followed:
points (HM): 'hoog65a.txt', x=1, y=2, v=3, X=4, radius=3000, min=2,
average;
variogram (HM): -100 Nug(0) + 150885 Gau(2469,63.0,0.432);
mask: 'afstand.map';
predictions (HM): 'tmp.prd';
variances (HM): 'tmp.var';

greetings,
Marlous