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Re: AI-GEOSTATS: Kriging with External Drift

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  • sshibli@mac.com
    ... There is a nice section (Chapter 5.4) in Cressie s textbook (Statistics for Spatial Data) that discusses the potential bias of estimating semivariograms
    Message 1 of 13 , Dec 7, 2001
      On Friday, December 7, 2001, at 10:05 AM, Andreas Hartmann wrote:
      >
      > How do I compute the residual semivariogramm when I don't know the
      > residuals? Or have I misunderstood something in the concept of external
      > drift?

      There is a nice section (Chapter 5.4) in Cressie's textbook (Statistics
      for
      Spatial Data) that discusses the potential bias of estimating
      semivariograms from residuals and Matheron's formulation of
      IRF-K kriging as an alternative means to krige in the presence of
      a trend. The iterative fitting of the covariance is usually
      non-graphical,
      which is a drawback in itself, and Fig. 5.1 in the same section shows
      Cressie's bold attempt at showing the pitfalls of doing such an
      "automatic" fit. I have not used IRF-K kriging very much in practice,
      for the above reasons. Better to live with the devil that I know, i.e.
      my -- albeit biased -- residual variogram.

      Regards,

      Syed



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    • Nicholas Lewin-Koh
      Hi, Just to add to that Gotway and cressie I can t remember the exact citation did a large simulation study and showed that the bias from using the residual
      Message 2 of 13 , Dec 8, 2001
        Hi,
        Just to add to that Gotway and cressie I can't remember the exact citation
        did a large simulation study and showed that the bias from using the
        residual variogram is very small in most cases.

        Nicholas

        On Fri, 7 Dec 2001 sshibli@... wrote:

        >
        > On Friday, December 7, 2001, at 10:05 AM, Andreas Hartmann wrote:
        > >
        > > How do I compute the residual semivariogramm when I don't know the
        > > residuals? Or have I misunderstood something in the concept of external
        > > drift?
        >
        > There is a nice section (Chapter 5.4) in Cressie's textbook (Statistics
        > for
        > Spatial Data) that discusses the potential bias of estimating
        > semivariograms from residuals and Matheron's formulation of
        > IRF-K kriging as an alternative means to krige in the presence of
        > a trend. The iterative fitting of the covariance is usually
        > non-graphical,
        > which is a drawback in itself, and Fig. 5.1 in the same section shows
        > Cressie's bold attempt at showing the pitfalls of doing such an
        > "automatic" fit. I have not used IRF-K kriging very much in practice,
        > for the above reasons. Better to live with the devil that I know, i.e.
        > my -- albeit biased -- residual variogram.
        >
        > Regards,
        >
        > Syed
        >
        >
        >
        > --
        > * To post a message to the list, send it to ai-geostats@...
        > * As a general service to the users, please remember to post a summary of any useful responses to your questions.
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        >

        CH3
        |
        N Nicholas Lewin-Koh
        / \ Dept of Statistics
        N----C C==O Program in Ecology and Evolutionary Biology
        || || | Iowa State University
        || || | Ames, IA 50011
        CH C N--CH3 http://www.public.iastate.edu/~nlewin
        \ / \ / nlewin@...
        N C
        | || Currently
        CH3 O Graphics Lab
        School of Computing
        National University of Singapore
        The Real Part of Coffee kohnicho@...


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