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Re: AI-GEOSTATS: bad regression between predicted and measured

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  • Isobel Clark
    Duccio There are many reasons why your interpolations may not be working. A few of these are: # you are beyond the range of influence of any distance
    Message 1 of 3 , Oct 13, 2003
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      Duccio

      There are many reasons why your interpolations may not
      be working. A few of these are:

      # you are beyond the range of influence of any
      distance relationship, that is you have too widely
      spaced sampling.

      # your data may have a skewed or other non-Gaussian
      distribution which makes both semi-variogram and
      correlation calculations invalid

      # you may have discontinuities, trends or anisotropies
      which have not been factored into your model

      Isobel
      http://drisobelclark.ontheweb.com

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    • Russell Barbour
      Dear Dr. Rocchini: Well, my first reaction is that R squared is not an appropriate measure for correlated data. Secondly, such a large area may well have an
      Message 2 of 3 , Oct 14, 2003
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        Dear Dr. Rocchini:

        Well, my first reaction is that R squared is not an appropriate measure for
        correlated data. Secondly, such a large area may well have an underlying trend
        which should be removed before kriging is attempted and lastly the number of
        observations seems very small if that is the only data layer available.

        Sincerely

        Russell Barbour Ph.D.
        Research Associate in Applied Mathematics
        Vector Ecology Laboratory
        Yale School of Medicine
        60 College St. Rm 600
        New Haven CT. 06520
        TEL: 203 785 2394
        FAX 203 785 3604
        email: russell.barbour@...


        Quoting rocchini@...:

        > I've made an interpolation by IDW, Spline and ordinary kriging and
        > a cross-validation for all methods.
        > When I make a regression between predicted and measured values I reach an R2
        > of 0.0109 (spline), 0.006 (kriging), 0.0006 (idw), namely very flat curves
        > (lines).
        > What could I do? I think I'm using a small number of input values (62) for
        > the area that I'm considering (4000 km2).
        > Thanks
        > Duccio Rocchini
        >
        > Dr. Duccio Rocchini
        > Dpt.of Env. Sci.
        > University of Siena
        > www.rocchini.net
        >
        >
        >
        >
        >
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