--- William Thayer <[EMAIL PROTECTED]> wrote: > Isobel: > Would you mind expanding a little on your earlier > reply? In particular, > what do you mean by (1) "kriging variance less than > ordinary > >statistical sample variance", (2) "not measured but > within the range > >of influence of at least 4 samples" and (3) > "indicated is within range but > >the local average is a better estimator than the > kriging"? > > Thanks in advance, > Bill Hi Bill It is possible for the kriging variance to be higher than the total sill of the semi-variogram. This total sill is (theoretically) equal to the Normal population variance if your data is (a) stationary and (b) Normal. For lognormals use logarithms. Or transform data to Normal scores before calculating semi-variogram. For example: if you estimate a point location from a single sample just below the range of influence away, the kriging variance is twice the total sill (unless you follow the Stanford school, in which case it is twice the total sill minus twice the nugget effect). Now, if the kriging variance is higher than the 'sample' variance, it means that the population mean (if you knew it) would be a better estimator than the local kriging estimate. So even if you have samples within the range of influence, you could still get an estimate which has worse confidence than the regional average. This I call "indicated". I'm sure it is there but I can't put a local value on it. If the kriging variance is less than the total sill (suitably modified for non-point support), then one can assign a 'local' value which is better than the regional average. I consider this "measured". Anything outside the range of influence is speculative and the province of the geologist. I am a mining engineer and don't do "inferred". Does this help? Isobel http://uk.geocities.com/drisobelclark PS: I have fond memories of Syracuse. Spent the summer of '76 as visiting prof in geology. ____________________________________________________________ Do You Yahoo!? Get your free @yahoo.co.uk address at http://mail.yahoo.co.uk or your free @yahoo.ie address at http://mail.yahoo.ie -- * 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
