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GEOSTATS: summary: data for variogram

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  • Sonja Brenner
    Dear all, sorry for not writing this earlier. This is a summary of the answers to my ... Andrew Liebhold wrote he wouldn t advise a variogram based on
    Message 1 of 1 , Oct 29, 1999
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      Dear all,
      sorry for not writing this earlier. This is a summary of the answers to my
      question:

      > can anyone tell me if I can use variogram
      > > estimation for a data set of only 30, 50 or 100 measurements? And what
      > > about qualitative data?

      Andrew Liebhold wrote he wouldn't advise a variogram based on < 100 points,
      because he doesn´t think that it is very reliable

      Andrew Lister wrote, he would trust it if one had at least 20 pairs of points
      in each lag
      class; it is possible to weight the variogram by the number of pairs/lag class

      You can use it for as little as 30 datasets - but it might be very erratic.

      Tony Lolomari wrote, one will just have to use some "soft" (qualitative)
      information to guide
      the sill and range parameters. Depending on what algorithm you're using
      (kriging, gaussian simulation, indicator simulation), you can get the
      theoretical sill value (=variance, = 1, = 1(1-p) respectively.

      Dragoljub Pokrajac wrote : According to my experience, it is not Ok to use
      variograms for 30 data. For 100, probably you can get something useful, but
      must regulate the number of variogram bins.

      Mark Murphy wrote, one can certainly try to evaluate the spatial continutiy
      with
      that many measurement. Depends a lot on the data spacing and the
      type of continuity one expects but he has done several exercises producing
      kriged estimates (using variograms) for as few as 30 data points. It is also
      possible to do categorical data by using indicator (ie yes/no coded to 1 or 0)
      coding to get an idea of spatial continuity.

      Carme Hervada i Sala wrote, that it is possible, but if you do them with less
      than 100 data, their models are not going to be quite reliable- you cannot be
      sure they represent all the population

      Åsa Danielsson mentioned to check this reference:
      Webster R. and M. Oliver (1992) 'Sample adequatly to estimate variograms of
      soil properties'. Journal of Soil Sciences 43: 177-192. They state you need at
      least 100 observations to get a reasonable variogram, but 250 would give a
      good one. (that´s really an interesting paper in my opinion).

      Heinz Burger wrote, it is a l w a y s possible to make a spatial analysis. If
      there isn´t an autocorrelation, it´s because of missing data in this distance.
      He told that a structure is to see with about 30 measurements. Qualitative
      data can be coded with Indicator-Transformation (look at: Deutsch&Journel:
      GSLIB)

      Edzer J. Pebesma finally wrote : Both is possible.

      Thank you again to all who answered!

      Sonja



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