Thanks everyone for the quick and accurate responses.

My original question

Can you please help me decipher the following message from S+ spatial

module...

> elev.krige.gaus <- krige(Z~loc(X,Y), data = elev, covfun=gauss.cov,

+ range = 90, sill = 115)

Error in .Fortran("ssukrige",: Covariance matrix is not positive

definite

Dumped

Warning messages:

Covariance matrix is not positive definite in: .Fortran("ssukrige",

.

I can get krige to work for a spherical model, but not for the

gaussian. Is this a problem with a large matrix (over 900 elements in

the elev data set) in S+?

The solution (2 parts) provided by Stephen Kaluzny, J. Kern, K.

Malakhanow, D. Allard, B. Ripley, A. Riding, D. Myers, and C.D. Lloyd:

The first is that since many of my values are close together S+ is

treating them as in the same area. Therefore I needed to increase the

number of values used in the covariance function interpolation. It turns

out that the "nc = " parameter controls this. I increased from the

default (1000) to 500000 (the most allowed by the default memory

settings).

Concurrent with this I also added a nugget value of 3 to my model. As

Donalsd Myers puts it:

"However the problem with the Gaussian is a further complication. If you

examine the graph of a Gaussian you see that it has values that are

almost

zero (assuming no nugget) for a considerable distance (relative to the

range) hence when one uses the usual kind of a search neighborhood,

i.e.,

only the nearest data locations, this can result in lot of variogram

values

in the kriging matrix that are nearly zero. This likely results in an

ill-conditionned matrix and depending on how they attempt to solve the

system of equations it may not have a unique solution."

But just increasing my nugget to 3 was not sufficient, therefore I

increased the nugget AND the nc value. Hope this helps ppl in the

future.

Thanks again...

Steven Pousty

--

*To post a message to the list, send it to

ai-geostats@....

*As a general service to list users, please remember to post a summary

of any useful responses to your questions.

*To unsubscribe, send email to

majordomo@... with no subject and

"unsubscribe ai-geostats" in the message body.

DO NOT SEND Subscribe/Unsubscribe requests to the list!