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Re: GEOSTATS: simulation with trend

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  • Pierre Goovaerts
    Hi Tom, This option is available in the latest version of Gslib. You can download the code at http://www.ualberta.ca/~cdeutsch/. Note that this program won t
    Message 1 of 2 , Dec 13, 1999
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      Hi Tom,

      This option is available in the latest version of Gslib.
      You can download the code at http://www.ualberta.ca/~cdeutsch/.
      Note that this program won't use the theoretically correct
      simple kriging variance if you select the option kriging
      with an external drift. The problem would then be that
      since the KED variance is larger than the SK variance,
      you will tend to have too much variability in the simulated
      maps that won't reproduce the target histogram very well
      after the normal score back-transform..
      Two options are:
      1. modify the program to solve at each
      node a KED and a SK system.

      2. simply multiply at each node the KED variance by a
      constant to approximate the SK variance. This constant can be
      derived by calibration: just run KED and SK on your normal score
      data using the same grid as for simulation and look at the
      ratio of the two variances SK/KED. Hopefully, it should be fairly
      constant across the grid. This ratio can then be inserted in Gslib
      source code by changing the command:
      sim(index) = xp * cstdev + cmean
      by
      sim(index) = xp * sqrt(ratio) * cstdev * constant

      Cheers,

      Pierre
      <><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><>

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      On Mon, 13 Dec 1999, Tom Charnock wrote:

      > Hi
      >
      > I've been looking at simulation, particularly in the presence of a trend
      > that can be described by an external drift.In the Gslib book (1992) they
      > show an example of realisations made using a modification of the sgsim code
      > but do not provide the code itself. They mention some problems of simulation
      > with a trend using a sequential gaussian algorithm (p276).
      >
      > In general what are the problems of simulation in the presence of a trend?
      > Are some algorithms better than others?
      > Does anyone know of any code available to do this?
      >
      > cheers Tom
      >

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