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[ai-geostats] Re: Positive Definite Problems

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  • Isobel Clark
    Dean Couple of possible explanations. Try these for size: (1) you may have points which are very close together if not with identical co-ordinates. Bear in
    Message 1 of 2 , Oct 25, 2004
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      Dean

      Couple of possible explanations. Try these for size:

      (1) you may have points which are very close together
      if not with identical co-ordinates. Bear in mind that
      'close together' depends on the precision of your
      software. Most software works to around 8 significant
      figures. If you co-ordinates are in the millions, the
      computer will not be able to see the difference
      between two samples at a distance less than (say) 1.
      First check your data for duplicate samples (most
      common occrrence!).

      (2) you may be using one of the semi-variogram models
      which is not terribly stable, like the 'hole effect'
      or the gaussian. The above problem intensifies if you
      use a gaussian, because 'too close' now means the
      whole of the initial curve on the model.

      (3) you may have an equation solving routine which
      loses precision very fast and be using too many
      samples.

      If none of these does it for you, I'll try to think of
      others!

      Isobel
      http://geoecosse.bizland.com/softwares
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