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GEOSTATS: variogram procedure of GSLIB

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    Dear List, I ve recently started using GSLIB and have been very pleased with my results. I am, however, sad that I don t understand exactly what s going on in
    Message 1 of 1 , Dec 13, 1999
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      Dear List,

      I've recently started using GSLIB and have been very pleased with my
      results. I am, however, sad that I don't understand exactly what's going on
      in the program (being fortran illiterate). The book (the first and second
      editions) don't seem to explain as much as I'd hoped. My impression from
      the example at the beginning of the book "A straightforward 2-d example" is
      that what should be done is to model the variogram in the directions of
      major and minor continuity (longest and shortest ranges of the variogram)
      and use this information in the parameter files. That is fairly clear; what
      I'm not clear about is the case where 1. the sills are not the same for the
      two variograms(zonal anisotropy I believe; the example in GSLIB doesn't seem
      very different from the geometric example), 2. where the variogram map is
      not exactly oval (the major and minor axes are not perpendicular to one
      another, which seems to be often assumed), and 3. where it seems that the
      "best fit" model does not have the same combination of structures (e.g.,
      spherical and exponential in the major direction, spherical and spherical in
      the minor).

      Furthermore, I don't quite see how programs like Sage and Surfer calculate
      anisotropy ratios (major/minor ranges of anisotropy); when using their
      procedure, you fit your model to all directions at once. It seems to me
      that to get a range parameter in a given direction, you would need to
      statistically fit a model in that direction. But from what I can tell, Sage
      and Surfer 7 somehow estimate these ranges based on the one
      "omni-directional" best-fitting model. This approach seems different from
      what is implied in the example in the GSLIB books.

      Finally, the anisotropy ratio returned in Surfer does not seem to be the
      ratio of the major range/minor range when I examine the directional
      variograms. The angles Surfer return seem correct, but the ratio of the
      ranges seem somewhat different. I really like the variogram procedure in
      Surfer 7, however....

      Lastly, I did a little experiment using my data. I fit a good-looking
      variogram, and then a really absurd one, with the same general shape, and
      found that the results of kriging (in Surfer) were strikingly
      similar....Perhaps I'm going overboard worrying about the precise
      form/anisotropy ratios? (probably a case by case situation)

      I feel that these are probably rather banal questions, but I searched
      through the old mail archives and didn't find any answers. Thanks for any
      suggestions/references/advice. I'll summarize....

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