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....

Sincerely,

Andy

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