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Re: AI-GEOSTATS: SA using GSLIB

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  • Pierre Goovaerts
    Dear Uwe, The interpretation of the nlag parameter in sasim can be somewhat confusing. It corresponds to the number of lags in all directions to be
    Message 1 of 2 , Jun 13, 2003
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      Dear Uwe,

      The interpretation of the nlag parameter in sasim
      can be somewhat confusing. It corresponds to the number
      of lags in all directions to be incorporated in the objective
      functions, not the class of distance for the semivariogram
      model. For example, to reproduce the variogram for
      the 1st "lag distance" of 0.5 km in all directions,
      you need to specify nlags=8, which corresponds to
      the number of cells in the direct neighborhood of the cell
      being simulated. For reproducing the 2nd class of distance
      of 1km, nlags should be 24. So the formula to use to
      reproduce K classes of distances would be
      nlag=[(2K+1)^2]-1. So if you want to reproduce the variogram
      up to a distance of 20 kms using a grid size of 0.5km
      nlag=[(2x40+1)^2]-1=6560! I have recently looked at ways
      to sample this set of lags since it is clearly impossible
      to afford such a large number of lags in the objective function.
      For example, only a random subset of these 6,560 possible lags
      would be reproduced. This is a limitation of the simulated annealing
      algorithm (would be even worst in 3D) and any other suggestions
      would be much appreciated.

      Regards,

      Pierre Goovaerts

      <><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><>

      Dr. Pierre Goovaerts
      President of PGeostat, LLC
      Chief Scientist with Biomedware Inc.
      710 Ridgemont Lane
      Ann Arbor, Michigan, 48103-1535, U.S.A.

      E-mail: goovaert@...
      Phone: (734) 668-9900
      Fax: (734) 668-7788
      http://alumni.engin.umich.edu/~goovaert/

      <><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><>

      On Fri, 13 Jun 2003, Uwe Haberlandt wrote:

      > Dear all,
      >
      > I have a question concerning the simulated annealing module within GSLIB
      > (f77v2 and f90). I'm using kilometers as units with grid spacing of 0.5
      > km and wanted a reproduction of the variogram up to a distance of 20 km.
      > The range is 10 km. However for different nlags set in the parameter
      > file (e.g. 20, 40, 80,120) the debugging file reports variogram values
      > (actual and model) much lower than expected. In the debugging file the
      > model variogram does not reach the sill not even with nlags=120. How is
      > "nlags" interpreted and what is reported in the debug file?
      >
      > Any help is greatly appreciated
      > Regards
      > Uwe
      >
      >
      > **********************************************************
      > Dr. Uwe Haberlandt
      > Ruhr University Bochum
      > Institute for Hydrology, Water Management
      > and Environmental Engineering
      > Universit├Ątsstra├če 150
      > 44780 Bochum
      > Germany
      > Tel.: +49 (0)234-32-27619
      > Fax.: +49 (0)234-32-14153
      > e-mail: uwe.haberlandt@...
      > www: http://www.ruhr-uni-bochum.de/hydrology/
      > **********************************************************
      >
      >


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