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842Re: AI-GEOSTATS: Experimental variograms.

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  • Tom Mueller
    Jan 13, 2003
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      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" <digbym@...>
      To: "ai-geostats" <ai-geostats@...>
      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

      digbym@...
      http://www.users.on.net/digbym


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      Tom Mueller
      Assistant Professor of Agronomy
      University of Kentucky
      Lexington, Ky 40546-0091
      859-257-8887
      mueller@...
      www.uky.edu/~mueller

      phone 859-257-8887
      cell 859-608-2215

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