Sebastiano Large kriging variances have nothing to do with negative weights. The two are completely different phenomena - especially in Universal Kriging.
In Ordinary Kriging the estimation variance can become as high as two times the total sill of the semi-variogram. This is a theoretical fact(?) and you can derive the answer by simply trying to estimate the value at one point from a single sample at or beyond the range of influence from this location. That is, a weight of +1!! In Universal Kriging, the weights often become negative because the system is trying to force the estimated point to lie on the trend of the samples. Sometimes it can only do this by using negative weights. There is no problem here, unless you have extremely high 'erratic' residuals. In this case, you should probably resolve that problem before trying kriging. If the variance becomes very high in Universal Kriging it is probably because you are extrapolating into sparsely sampled areas. Remember you are trying to estimate the trend as well as the 'residual' value and this contributes to higher variances. You should widen your search radius to include more samples than with Ordinary Kriging. You can see how all this works with our free kriging game. If your data is in Geo-EAS form or a simple CSV file, you can read it into Krigame and see the equations and the solutions. Then vary search parameters etc to see how they are affected. You might want to download our Tutorial 3 which discusses Universal Kriging with the Wolfcamp data. All available at http://geoecosse.bizland.com/softwares with no restriction on use or distribution. Isobel ________________________________________________________________________ Want to chat instantly with your online friends? Get the FREE Yahoo! Messenger http://mail.messenger.yahoo.co.uk -- * To post a message to the list, send it to [EMAIL PROTECTED] * As a general service to the users, please remember to post a summary of any useful responses to your questions. * To unsubscribe, send an email to [EMAIL PROTECTED] with no subject and "unsubscribe ai-geostats" followed by "end" on the next line in the message body. DO NOT SEND Subscribe/Unsubscribe requests to the list * Support to the list is provided at http://www.ai-geostats.org
