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RE: AI-GEOSTATS: Kriging with External Drift

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  • Isobel Clark
    ... you got it. Plus the constraint that the drift components in the samples have to balance with the drift component at the point being estimated. Isobel
    Message 1 of 13 , Dec 7, 2001
      > So are you saying that the external drift variable
      > in the matrix is just the
      > magnitude of the the drift variable at that point?
      you got it.

      Plus the constraint that the drift components in the
      samples have to balance with the drift component at
      the point being estimated.

      Isobel

      ________________________________________________________________
      Nokia 5510 looks weird sounds great.
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      The competition ends 16 th of December 2001.

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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 2 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 3 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@...
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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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