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1017AI-GEOSTATS: Log normal kriging (2)

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  • Gregoire Dubois
    May 13, 2003
      Dear all,

      my last posting on lognormal kriging was either stupid, tricky or unclear but
      I didn't get any reply to it. Were the readers shocked that I planned to do a
      biased back transformation ? :)

      If one is browsing the literature presenting case studies involving lognormal
      kriging, one would be amazed by the lack of information the authors provide on
      the back transformation of their estimates. I simply cannot believe that the
      authors used the unbiased back transformation (see the correct transformation
      in Journal and Huybregts, 1989, p. 572 or in Saito, H. and P. Goovaerts. 2000.
      Geostatistical interpolation of positively skewed and censored data in a
      dioxin contaminated site. Environmental Science & Technology, vol.34, No.19:
      4228-4235).

      The unbiased back transformation involves the kriging standard deviation as
      well as the Lagrange parameter. This renders the transformation rather tricky
      (hence my plan to use a biased transformation). Since the Lagrange parameter
      is not provided in the output files, I wonder how the authors who have
      published case studies with lognormal kriging did their transformation (a few
      case studies were even done with software that were not providing the kriging
      variance). Are there any free/cheap geostats package I'm not aware off that
      are performing log-normal kriging and the unbiased back transformation? (there
      are none listed on the AI-GEOSTATS web site).

      I also saw case studies where the back transformation did not take the
      Lagrange parameter into account. How can this be justified and how much would
      such an approach affect the transformation?

      Thanks for any clarification about this matter.

      Best regards,

      Gregoire

      PS: as a colleague told me, geostatistics is often closer to black magic than
      to any scientific discipline...



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