Hi Digby: In a paper (Mueller, T.G., F.J. Pierce, O. Schabenberger, and D.D. Warncke. 2001. Map quality for site-specific management. Soil Sci. Soc. Am. J. 65: 1547-1558), I suggest that the variance of the semivariogram at each lag may explain why map quality was poorer than expected in this study. I also suggest that this variance, particularly at earlier lags may be a good indicator of map quality. Bottom line is that semivariogram models are still just field average models of spatial autocorrelation. Since its not a Gaussian distribution (more like a Chi square distribution) it may not be appropriate to use CV’s do describe the semivariogram cloud. Something I’ve been looking at recently is Hawkins and Cressie’s robust estimator. It transforms the semivariogram distribution into a normal distribution to calculate the semivariogram and then rescales it back. I’ve found RMSE values are smaller with this approach more often than not with jack-knife analysis.
Tom -----Original Message----- From: "Digby Millikan" <[EMAIL PROTECTED]> To: "ai-geostats" <[EMAIL PROTECTED]> Date: Mon, 13 Jan 2003 15:35:40 +1030 Subject: AI-GEOSTATS: Experimental variograms. Hello, When one calculates an experimental variogram used for spatial modelling one averages the semivariogram values at each distance. This value is just an average with a certain variance and coefficient of variation which we assume to represent the true underlying variogram as we only have samples of the entire population. To this effect can anyone suggest any literature that which discusses individual variogram variances and the effect of these assumptions, and smoothing it may cause in our modelling, or is that our averaged estimates of the variogram are accurate represenations as in many cases they do produce smooth curves. Thanks in advance. Regards Digby Millikan B.Eng Geolite Mining Systems U4/16 First Ave., Payneham South SA 5070 Australia. Ph: +61 8 84312974 [EMAIL PROTECTED] http://www.users.on.net/digbym -- * 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 Tom Mueller Assistant Professor of Agronomy University of Kentucky Lexington, Ky 40546-0091 859-257-8887 [EMAIL PROTECTED] www.uky.edu/~mueller phone 859-257-8887 cell 859-608-2215 -- * 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
