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AI-GEOSTATS: basic theoretical question

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  • Koen Hufkens
    I have had no courses in geostatistics what so ever but I m trying to calculate the semivariance of a dataset on my own. Calculating everything from scratch.
    Message 1 of 2 , Dec 16, 2003
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      I have had no courses in geostatistics what so ever but I'm trying to
      calculate the semivariance of a dataset on my own. Calculating
      everything from scratch.

      But I have some basic questions conserning the calculations that have to
      be done.

      First of all, I have data that is stratified on one level but not so on
      another. a sample plot would look like this:

      x x x
      x x
      x x x
      x x
      x x x

      a few of these sample sites were randomly distributed in a grid pattern
      of 9x9 km with grid cells of 1 km, 5 sites per cell.

      so I have about +- 500 points sampled. (is this enough??)

      So goes my question, is semivariance a good idea? Given the fact that a
      sampling plot is 20x20m and the whole site 9x9km? Does it make sense to
      include all values or should I only consider the 1x1km grid cells as
      maximum. In the end, correlations between points in 1 site will be long
      lost if compared to the ones in the other plots. Any idea's on this.

      Anyway, if I go with calculating the semivariance for the whole 9x9km
      what should I do. I made a flowchart of what should be considered this
      is what I came up with.

      first, calculate the distances between the pairs of sample points, and
      the square of the differences between the attributes assigned to these
      points.

      If I have these points, because the site has no random distribution and
      in general there are not that many points that have the same distances
      you will have to reclass these distances and associated attributes into
      classes to have your lag distances. From what I have read this h
      distance may not be more then half the maximum distance between two
      points.

      This would leave me with lags, h, from the reclassed distances the
      squared difference of attribute values and a count of observations of
      lags per class (N).

      As far as I know this are all the necessary values needed to calculate
      the semivariance with following equation:

      gamma(h)=1/2N(h)*sum((z(x)-z(x+h))^2

      Am I'm doing this right???
      Any input on this matter would be greatly appreciated.

      Thank you for your time,
      Koen Hufkens


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    • Isobel Clark
      Hi Koen Firstly, I have to tell you that many people would kill for the type of data you have - it is absolutely ideal for trying out a geostatistical
      Message 2 of 2 , Dec 16, 2003
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        Hi Koen

        Firstly, I have to tell you that many people would
        kill for the type of data you have - it is absolutely
        ideal for trying out a geostatistical approach.

        Secondly, can I suggest that you work through our free
        tutorials - especially Tutorials 2 and 3 - which can
        be downloaded from
        http://geoecosse.bizland.com/softwares in Word format.
        If you need pdf or some other format, please let us
        know. These Tutorials take you from first look at data
        through to kriging in a fairly straightforward manner
        with a tutorial data set (which is also downloadable).

        Please let me know if you have any problems with this,
        or further questions.

        Isobel [Clark]
        http://geoecosse.bizland.com/whatsnew.htm


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