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Re: AI-GEOSTATS: Universal Kriging in ArcInfo

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  • Patrick van Laake
    Dear listmembers, I tried to send this before, but I don t think anybody but Jason has seen it. If you have, sorry for the inconvenience. (List Manager, maybe
    Message 1 of 4 , Jan 17, 2001
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      Dear listmembers, I tried to send this before, but I don't think anybody but Jason has seen it. If you have, sorry for the inconvenience. (List Manager, maybe it is possible to put ai-geostats@... first and
      then the author such that a simple reply to a post works?)

      >Hello
      >
      >This is my first time on the list and I hope this question is going to
      >the right place. My appologies if not.
      >
      >I am using Arc/Info 7.2.1 on an NT workstation.
      >With GRID I am using linear universal kriging to produce maps of heavy
      >metal concentration from 127 sample points.

      In response to the replies of Isobel Clark and Martin Rosevaere and as a
      general warning to yourself: Yes, ArcInfo does automatically produce a
      semi-variogram and you can specify its functional form (the {method} optional
      parameter). Beware, however, of the method that is applied to parameterize the
      variogram. As is stated in the help file, the Levenberg-Marquardt (LM)
      algorithm is applied. LM is a non-linear least squares method of function
      estimation from sampled data. It works by minimizing chi square over the
      sampled data >>>> using the standard deviations of the individual samples
      <<<<. ArcInfo kriging does not ask for, therefore does not know of nor use the
      standard deviation of the sample data (your point cover). Instead, and this
      completely invalidates ArcInfo kriging IMHO, you specify either a number of
      neighbouring sample points or a radius from which to pick sample points to
      calculate local sample standard deviation. THIS IS COMPLETELY AND ABSOLUTELY
      WRONG!!!!! It undermines the essence of kriging. Do not use kriging in ArcInfo
      if you are trying to do serious work, or if your sample data is of high
      quality. Use a proper tool instead.

      And to conclude an excerpt from ArcInfo help on kriging:

      [quote]
      Kriging is a complex procedure that requires greater knowledge about spatial
      statistics than can be conveyed in this command reference. Before using the
      KRIGING command, you should have a thorough understanding of the fundamentals
      of kriging and have assessed the appropriateness of your data for modeling
      with this technique. If you do not have a good understanding of this
      procedure, it is strongly recommended that you review some of the references
      listed at the end of this command reference.
      [unquote]

      If you have a good understanding of kriging and KRIGING, don't use ArcInfo!

      Patrick van Laake
      ESRI Certified ArcInfo Instructor (really but ;-) )



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