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Re: AI-GEOSTATS: Sills of Directional Correlograms

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  • L. Wiles
    ... Thank you for the responses. I would like to add some information in case it might be helpful: -My data is highly skewed with usually 50% zero counts.
    Message 1 of 3 , Feb 26, 2001
      My original question was:
      > What is the interpretation of a directional correlogram that does not
      > reach the sill?

      Thank you for the responses. I would like to add some information
      in case it might be helpful:

      -My data is highly skewed with usually >50% zero counts. It is not
      bi-modal or multi-modal. I did a ln+1 transformation before analysis.
      -Residuals after detrending the data shows the same pattern of not
      reaching a sill in one direction (There was never much of a trend).
      -Variograms also show the same pattern of not reaching a sill in one
      direction.
      -I have looked at postings of my data over and over and do not see
      any consistent pattern for all the distributions that do not reach the sill
      in one direction.

      Additional questions: Is there a more specific interpretation than
      "non-stationarity?" If it is zonal anisotropy, what sort of phenomena
      lead to zonal anisotropy?

      Thank you.



      > Details:
      > I'm using correlograms to describe the spatial distribution of weed
      > seed banks within agricultural fields. I sampled on a square grid and
      > I'm calculating correlograms for every 45 degrees. In many cases, the
      > correlogram does not reach a sill of one in just one direction. The
      > sill is one in the other directions.
      >
      > It is obvious from a posting of the data that the range of spatial
      > dependence in that direction is much smaller than the maximum
      > calculated distance of the correlogram. Also, besides not reaching
      > the sill, this directional correlogram exhibits typical behavior of
      > the correlograms in my study that do reach a sill of one. The
      > direction that does not reach the sill is the same in most of the
      > fields (direction has biological meaning in this study).
      >
      > In a couple of cases, the seed bank appears to have two different
      > spatial distributions within a field with the division parallel to
      > direction of the correlogram that does not reach a sill of one. When
      > I split my data along the division, the directional correlograms for
      > each part reach a sill of one for all directions. However, the
      > remaining fields appear to have a consistent spatial distribution
      > within the field and any division would be arbitrary.
      >
      > I would appreciate help with interpretation.
      >
      >
      > ------------------------------------
      > Lori J. Wiles
      > Weed Ecologist
      > USDA-ARS-WMU
      > AERC - Colorado State University
      > Fort Collins, CO 80523
      > Phone: 970-491-8520
      > FAX: 970-491-8247
      > email: wiles@...
      > ------------------------------------
      >
      >
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      ------------------------------------
      Lori J. Wiles
      Weed Ecologist
      USDA-ARS-WMU
      AERC - Colorado State University
      Fort Collins, CO 80523
      Phone: 970-491-8520
      FAX: 970-491-8247
      email: wiles@...
      ------------------------------------


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