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  • Marc Van Meirvenne
    May 4, 2000
    • 0 Attachment
      Dear list,

      For those of you who use GAMV (Fortran 77 DOS version) of GSLIB (1998
      version) to calculate indicator variograms it is worthwhile to know that
      GAMV uses a different indicator coding from the one described in its
      manual, as we recently realized.

      In the manual, p. 11, Eq. II.6 defines the indicator I(u,z) = 1 if Z(u)
      <= z else I(u,z) = 0, with z being the cutoff value in respect to which
      the indicator is created. This is the most common definition of
      indicator coding, see also Goovaerts, 1997, p. 285, Eq. 7.20.

      However, GAMV is programmed to code the inverse way, but not exactly. On
      lines 323-327of GAMV, it says :

      if (vr(id,iv).lt.cut(ic)) then vr(id,jv) = 0.0 else vr(id,jv) = 1.0,

      or : I(u,z) = 0 if Z(u) < z else I(u,z) = 1. An indicator variogram will
      be the same regardless whether a 0 or a 1 is used to code values above
      or below the cutoff. But a difference from the definition in the manual
      occurs whenever an observation is equal to the cutoff, i.e. not the same
      observations will be coded equally according to the definition in the
      manual or according to GAMV. E.g. if the cutoff is 5, observations of
      value 4 and 5 both will be coded as a 1 according to the definition but
      they will be coded as 0 and 1 respectively in GAMV. Consequently the
      resulting indicator variograms will be different.

      GAMV is erroneous in the situation where one has a data set with a large
      proportion of zero's, say 30 %. In this case, when one uses zero as a
      cutoff, GAMV returns no indicator variogram (all indicator semivariances
      are zero) because all observations are coded as one ! Whereas 30 %
      should be coded as either one or zero and the remaining 70 % as the
      opposite (zero or one, resp.), yielding indicator semivariances > 0.

      In short, on line 323 of GAMV, the .lt. should be either .le. or .gt.,
      the latter being the preferred modification since it results in a coding
      identical to the definition given in the book.

      I hope this is helpful.

      Marc Van Meirvenne
      Dept. Soil Management and Soil Care
      Fac. of Agricultural and Applied Biological Sciences
      Ghent University
      Coupure 653, 9000 Gent, Belgium
      Tel. + 32 (0)9 264 6056
      Fax + 32 (0)9 264 6247
      e-mail : Marc.Vanmeirvenne@...

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