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  • andrew lister
    Dear group, I am a master s student in forest ecology and am a low intermediate geostatistics user. I have collected soil samples on 5 plots on an irregular
    Message 1 of 1 , May 28, 1998
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      Dear group,

      I am a master's student in forest ecology and am a "low intermediate"
      geostatistics user. I have collected soil samples on 5 plots on an
      irregular grid of points. Each 50x50m plot is separated by at least 75
      meters. I hope to, among other things, characterize the spatial patterns
      of several soil factors measured from each sample using variogram modeling.
      I have been using the GS+ software and am quite pleased, but have a few
      questions:

      1. stationarity of the data: I have read a number of articles in the
      ecology and agronomy literature discussing this concept, and have read some
      conflicting things (one researcher refers to the concept of stationarity as
      "troubling"). One article (by Jongman et al. I believe) says that
      nonstationarity of the variance leads to a meaningless expression of the
      autocorrelation function, but not of the semivariogram; with the variogram,
      only nonstationarity of the mean is a concern. Several articles say that
      both nonstationarity of the variance and mean need to be fixed before
      variogram modeling. Which is it?

      2. checking for stationarity: I just finished reading a paper by Hamlett,
      Horton and Cressie which shows some exploratory techniques for variogram
      analysis. They recommed doing local variance vs mean plots to check for
      stationarity of the variance. However, Isaaks and Srivastava's book
      mentions that this does not necessarily indicate nonstationarity, but a
      "proportional effect", something which happens in non-normally distributed
      data. In addition, the Hamlett et al paper seems to check for stationarity
      of the mean by simply looking at the stem and leaf plot and checking its
      symmetry, as well as looking at how the local mean changes across the
      sampling area. Indeed, many of the papers and books I've read say "the
      local mean should not change". Obviously, this is not literal, but nobody
      seems to say just how much change is ok for your data to be stationary, and
      likewise with the variance. How much change in local mean and variance is
      ok?

      3. trend removal: It's tempting just to say my data are non stationary and
      do median polish and do my variograms with the residuals. Is this a valid,
      albeit "black box", approach?

      4. When I finally do arrive at my variogram models, I would like to use
      them to block krig. I know that one approach is to do "universal kriging",
      for which I unfortunately do not have the software nor the expertise.
      Could I simply fit a first or second order trend surface to the data, model
      the variogram with my residuals, krig using ordinary kriging, and then add
      the trend back in at the end as is suggested in the "final thoughts" of
      Isaaks and Srivastava's book and elswhere (unless I am misinterpreting what
      I read)? That would make me very happy.

      5. Speaking of this, I wonder about the merits of obtaining the residuals
      for the variogram modeling from a 1st or 2nd order trend surface, vs.
      obtaining them from median polish?

      Thank you for your time; I will summarize any responses and post them to
      the list.

      Andrew Lister
      alister@...

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