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AI-GEOSTATS: Standardised versus general relative semivariograms ?

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  • Gregoire Dubois
    Dear all, can anyone point the relative advantages and drawbacks of - standardised semivariograms (See Pannatier, 1996: Variowin. Software for spatial data
    Message 1 of 3 , Mar 30, 2001
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      Dear all,

      can anyone point the relative advantages and drawbacks of

      - standardised semivariograms (See Pannatier, 1996: Variowin. Software
      for spatial data analysis in 2 D, page 39) where gamma(h) is divided by the
      variance for each lag;

      and the

      - general relative semivariogram (Isaaks & Srivastava, 1989: An introduction
      to applied geostatistics, page 164) where gamma(h) is standardised by the mean
      of each lag.

      I�m currently analysing the spatial structure of radioactive deposition for
      different levels with the help of indicators. Standardised & general relative
      semivariograms describe very well the structures while the semivariogram is
      not really appropriate for such a highly
      skewed variable. For low values of the chosen thresholds, the standardised
      semivariograms shows me a stronger spatial correlation compared to the general
      relative semivariogram.

      For higher threshold values, the opposite situation appears.

      Would this mean that low values show strong fluctuations but that the mean
      value remains quite constant in space while high levels of radioactivity show
      less fluctuations but the mean values change more in space ?. Has anyone
      experienced similar observations with other variables ?

      Apparently, there has not been much published on these functions, even if
      these are frequently used.

      Best regards

      Gregoire


      Gregoire Dubois (Ph.D.)
      Institute of Mineralogy and Petrography
      Dept. of Earth Sciences
      University of Lausanne
      Switzerland

      http://www.ai-geostats.org

      ____________________________________________________________________
      Get free email and a permanent address at http://www.netaddress.com/?N=1

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    • Horacio J. Buscaglia
      Hi, I would like to invite everybody to express their opinion respect the misconceptions pointed out in the paper Random sampling or geostatistical modeling?
      Message 2 of 3 , Mar 30, 2001
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        Hi,
        I would like to invite everybody to express their opinion respect the
        misconceptions pointed out in the paper Random sampling or geostatistical
        modeling? Choosing between design-based and model-based sampling
        strategies for soil. D.J.Brus and J.J. de Gruijter Geoderma 80 (1997) 1-
        44. The aim of this paper is to initiate a discussion on the role of
        design-based and model-based sampling strategies in soil science. The
        paper stress the Misconception 1:`Independence of sample data is
        determined by the physics of the phenomenon being sampled'. Misconception
        2:`Classical sampling theory assumes that sample data are independent'.
        Base on these statements, many questions could be asked regarding what is
        a `correct' or `incorrect' statistical analysis. Would it be correct to
        calculate correlation coefficients between variables without taking into
        account spatial correlation (ex. Mantel test)? A similar question could
        be: Are regression models correct in presence of spatial correlation or
        they are not correct and an autoregression model should be used?
        Thanks in advance to all of you for participating in the discussion,
        Horacio



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      • Ulrich Leopold
        Dear Gregoire, there exists a publication which could be interesting for you: Srivastava, R.M.; Parker, H.M. 1989: Robust measures of spatial continuity. In:
        Message 3 of 3 , Apr 3 3:47 AM
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          Dear Gregoire,

          there exists a publication which could be interesting for you:

          Srivastava, R.M.; Parker, H.M. 1989: Robust measures of spatial
          continuity. In: Armstrong, M. 1989 (Ed.): Geostatistics. Vol. I.
          Dordrecht, pp. 295-308.

          On 2001.03.30 14:23:44 +0200 Gregoire Dubois wrote:
          > Dear all,
          >
          > can anyone point the relative advantages and drawbacks of
          >
          > - standardised semivariograms (See Pannatier, 1996: Variowin. Software
          > for spatial data analysis in 2 D, page 39) where gamma(h) is divided by
          > the
          > variance for each lag;
          >
          > and the
          >
          > - general relative semivariogram (Isaaks & Srivastava, 1989: An
          > introduction
          > to applied geostatistics, page 164) where gamma(h) is standardised by
          > the mean
          > of each lag.
          >
          > I’m currently analysing the spatial structure of radioactive deposition
          > for
          > different levels with the help of indicators. Standardised & general
          > relative
          > semivariograms describe very well the structures while the semivariogram
          > is
          > not really appropriate for such a highly
          > skewed variable. For low values of the chosen thresholds, the
          > standardised
          > semivariograms shows me a stronger spatial correlation compared to the
          > general
          > relative semivariogram.
          >
          > For higher threshold values, the opposite situation appears.
          >
          > Would this mean that low values show strong fluctuations but that the
          > mean
          > value remains quite constant in space while high levels of radioactivity
          > show
          > less fluctuations but the mean values change more in space ?. Has anyone
          > experienced similar observations with other variables ?
          >
          > Apparently, there has not been much published on these functions, even
          > if
          > these are frequently used.
          >
          > Best regards
          >
          > Gregoire
          >
          >
          > Gregoire Dubois (Ph.D.)
          > Institute of Mineralogy and Petrography
          > Dept. of Earth Sciences
          > University of Lausanne
          > Switzerland
          >
          > http://www.ai-geostats.org
          >
          > ____________________________________________________________________
          > Get free email and a permanent address at http://www.netaddress.com/?N=1
          >
          > --
          > * To post a message to the list, send it to ai-geostats@...
          > * As a general service to the users, please remember to post a summary
          > of any useful responses to your questions.
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          > "unsubscribe ai-geostats" followed by "end" on the next line in the
          > message body. DO NOT SEND Subscribe/Unsubscribe requests to the list
          > * Support to the list is provided at http://www.ai-geostats.org
          >
          >

          --
          Ulrich Leopold
          Dipl. Geograph

          Engelstrasse 104
          D-54292 Trier
          Germany

          Phone: +49-651-140764
          E-mail: leop6101@... or uleopold@...
          URL: http://www.geocities.com/leop6101/index.htm


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