The replies I received to my request appear below, along with the original request for help. Thanks to those who replied.
Original request: I am interested in comparing different estimators of spatial means. Any suggestions or approaches on how to generate a 2-D, autocorrelated, skewed distribution that exhibits non-stationary mean and variance? Replies: How about using sasim.f in GSLIB to generate several non-conditional realizations of a property using simulated annealing? You can specify: 1. a user-defined histogram, which may be as skewed as you wish, and 2. a non-stationary power law variogram (fractional Brownian motion) to approximate a variable with a drift component. Hope this helps. Syed Bill, just a quick idea. Build a variogram with a trend in it, no sill, and use it in a Gaussian simulator (e.g., sgsim). Make the simulation in standard normal space and then use the GSLIB trans program to transform it to any raw-space skewed distribution you want. The transformation is quantile preserving so should not change the autocorrelation, but I would double-check the results. This process will certainly generate a correlated field with a skewed distribution and non-stationary mean. I'm not exactly sure how you want the variance to be non-stationary and that may be harder to do. Non-linear transforms can produce a prorportional effect (variance is a function of the simulated value), but they generally don't preserve the variogram. good luck Sean Without giving it too much thought, I wonder if just generating a stationary autocorrelated normal field, Zn and "back-transforming" it to produce a lognormal field, Zl wouldn't work? Because the kriging variance comes into both the mean and variance of the back-transformed variable, it should be non-stationary. Yetta Yes. Generate a Gaussian random field, add a deterministic trend surface, and take the exponent or a power transform of the sum. Edzer ************************************************** William C. Thayer, P.E. Environmental Science Center Syracuse Research Corporation 301 Plainfield Road, Suite 350 Syracuse, NY 13212 phone: (315) 452-8424 fax: (315) 452-8440 email: [EMAIL PROTECTED] web: http://esc.syrres.com/ http://esc.syrres.com/geosem/ ************************************************** -- * 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
