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1707Re: [ai-geostats] A question on lag class and lag distance

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  • Isobel Clark
    Oct 1, 2004
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      xhy

      your questions are long-standing and as yet unanswered
      in general.

      > 1. How to select the lag class and lag distance in
      > order to obtain a more reasonable experimental
      > variogram?
      I always think of it as focussing a camera. Believe
      there is a pattern in your data and our task is to
      balance 'width of interval' versus 'number of pairs in
      interval' to get the clearest picture.

      One of the things I have found most useful with
      irregularly spaced data is a 'nearest neighbour'
      analysis. Take each sample and find the closest one to
      it. Record the distance. Repeat for all samples. This
      process takes twice as long as calculating the
      semi-variogram but gives you an idea of the 'natural'
      or model spacing between your samples. This can be
      used to guide your choice of interval.

      Check out our free tutorial downloads at
      http://geoecosse.bizland.com/softwares

      > 2. Is it reasonable to use an uneven set of lag
      > (e.g. the lag increments are: 0-2.5m, 2.5-5.0m,
      > 5.0-12.0m, 12.0-19.5m, 19.5-27.0m, 27.0-30.0m,
      > 30.0-40m, 40-50m etc.) if a more stable variogram
      > can be obtained?
      I am not sure I have ever seen this done, but don't
      see why not if you plot the point at the centre of
      gravity of your interval (i.e. average distance of
      pairs found).

      Hope this helps
      Isobel
      http://geoecosse.bizland.com/books.htm





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