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Re: AI-GEOSTATS: Number of data points & Variograms

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  • Isobel Clark
    ... Hi, some thoughts (your numbering): (1) One of the things I have found successful is the following: construct your semi-variogram using ALL of your data
    Message 1 of 2 , May 15, 2001
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      > My research is on heavy metal pollution in water
      > bodies.........

      Hi, some thoughts (your numbering):

      (1) One of the things I have found successful is the
      following:
      construct your semi-variogram using ALL of your
      data but not allowing pairs between samples in
      different water bodies;
      use cross validation on each water body separately
      to see if the 'generic' model works for all of them or
      whether some are more variable or harder to predict
      than others;
      use the generic model for kriging with a
      variance/sill scaled for each water body.

      > Is there any consistent tested way to approach such
      > 'not-enough-data' situations?
      Not really, but I have found this works if the
      'deposition' is similar in the various bodies.

      (2) 'Hot spots' are (a) erratic highs due to
      distribution being skewed or (b) true outliers
      (inhomogeneities). Which? Tackle accordingly. Cross
      validation will pick up outliers but not work properly
      if data is severely skewed.

      > Could it be possible to effectively fit variograms,
      > when the hot spots are present?
      Try calculating semi-variograms with and without 'hot
      spots' and see what happens.

      Kriging is based on an assumption of homogeneity and
      it is a little unfair to expect it to come back and
      say "that's a daft thing to do" ;-)

      > [ For most of the cases I tried with such suspected
      > hotspot data, my results show that
      > the linear interpolation works better than the
      > krigged distribution based on the 'fitted'
      > variograms]
      I find this statement interesting. How do you define
      "better" -- prettier? nicer? easier to interpret? less
      polluted?


      Isobel Clark


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