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AI-GEOSTATS: Lag - Distance, Variogram / Semivariogram

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  • Ursula Manns
    Hi there, can anybody here tell me how to choose the correct (appropriate) lag distance for a regular sampling grid for orchids of 5.5 x 20 metres (Counting
    Message 1 of 2 , Sep 4, 2003
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      Hi there,

      can anybody here tell me how to choose the correct (appropriate) lag
      distance for a regular sampling grid for orchids of 5.5 x 20 metres
      (Counting data at a certain quadrate) ? I have problems in variogrphy,
      because lag distances of 5 metres seem to be problematic because of the
      regularity of the sampling grid and the missing values inbetween the 20
      meteres distances. If I choose a 15 metres lag I get a much smoother
      variogram. Choosing a 5 metres lag, the variance values are too high in the
      frist 3 -4 lags.

      My second question is what is the difference between a variogram and a
      semivariogram ? Is there any ? Is a correlogram easier to interprete ? I
      found it much easier to deal with values of my diagram between 0 and 1
      rather than having trouble to interprete a particular variance x !!??

      Hopefully you can help a newbie.

      Ursula:-)

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    • Isobel Clark
      Ursula ... This is possibly because you have some competition effect between orchids. For a square grid, we usually recommend an interval 20% of the grid
      Message 2 of 2 , Sep 4, 2003
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        Ursula

        > 5.5 x 20 metres
        > Choosing a 5 metres lag, the variance
        > values are too high in the first 3 -4 lags.
        This is possibly because you have some competition
        effect between orchids.

        For a square grid, we usually recommend an interval
        20% of the grid spacing, so you don't get diagonals
        lumped in with 'straight' directions. This would
        suggest that you should try a 1 metre lag, which is
        probably overkill.

        The other alternative is to construct directional
        semi-variograms and specify the correct lag for each
        direction, to see what differences you get.

        > My second question is what is the difference between
        > a variogram and a semivariogram ?
        As a general rule a "variogram" is a semi-variogram
        constructed by someone lax in their terminology. No
        software I know calculates the true variogram (twice
        the semi-variogram).

        The correlogram is simply the semi-variogram upside
        down and standardised to vary between -1 and +1. The
        disadvantage of this approach (or the covariance
        function is that it is difficult to assess the nugget
        effect accurately.

        You should be concerned about your variance as it
        provides essential information about teh variability
        of your phenomenon.

        Isobel Clark
        http://ecosse.ontheweb.com/whatsnew.htm

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