Mike, What you are dealing with is universal kriging, i.e., kriging in the presence of a drift. The data set that took away my geostatistical virginity was of this same type - the Wolfcamp aquifer data. Since the original 1985 geostat pub on this by myself and Jeff Furr, many folks have analyzed this data in numerous ways. The data is downloadable from http://uk.geocities.com/drisobelclark/briefcase.html#PG2000_demo. Analyze it with any software that allows universal kriging (e.g., a teaching version of the software on this same page).
In essence, estimate the trend but keep in mind that universal kriging may not need as high of a trend when fitting local search areas as a global regression trend over your entire data set. Thus while a quadratic regression may "best" fit your full data set, you might only need a linear trend when doing universal kriging since you are doing this in each smaller local search area. With the appropriate software you are in essence taking out the trend, and fitting the semi-variogram to the residuals. But when you then do the universal kriging after deciding on a reasonable semi-variogram, it will be done on the original data then using both the semi-variogram and trend you decided upon. See Practical Geostatistics 2000 (ISBN 0-9703317-0-3 or 0-9703317-2-X) for additional details on how one might approach this. I hope I am not violating any AI-GEOSTATS bylaws here. I am trying to give an answer that works for me without getting into any hot water. Bill Michael Dennis wrote: > Hello all, > > I'm a bit of a rookie with GeoStatistics. I am interested in Kriging with > External Drift but I am having a hard time finding information that tells > you how it works in laymans terms (without just firing a matrix at you and > leaving you to deduce what it means). > > I don't think this is right but I'll give it a shot. Does it work as > follows : > > 1) Compute a trend for the drift variable > 2) Remove the trend computed in 1) from the main variable > 3) Grid the residuals from step 2) > 4) Add back the trend from step 1) > > I don't think this is right but if you can explain to me how the Drift is > actually applied in laymans terms it would be greatly appreciated. Also > when you do kriging with external drift do you have to model a variogram or > can a reasonable one be computed automatically, if so how would you compute > it? > > Also if you have any good sources of information on Kriging with external > drift could you pass them on to me? > > Thanks, > > Mike > > -- > * 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 -- William V Harper, Mathematical Sciences, Otterbein College Towers Hall 136, Westerville OH 43081-2006 USA 614-823-1417 Fax 614-823-3201 Faculty page: http://go.to/billharper For the best in geostatistics: http://go.to/geostatistics Coming eventually: http://www.wvharper.com (currently points to geostat site) -- * 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
