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Re: [AI-GEOSTATS: Different Spatial Lag tolerance?]

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  • Gregoire Dubois
    Dear Rubens, removing pairs from a semivariogram is OK if you can justify the reason why you are doing so. One usually remove pairs because they influence the
    Message 1 of 1 , Jan 25, 2001
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      Dear Rubens,

      removing pairs from a semivariogram is OK if you can justify the reason
      why you are doing so. One usually remove pairs because they influence
      the spatial structure of the analysed phenomenon in an unexpected way
      (i.e. a �jump� in the nugget effect). However, in such a case one will realise
      that more than pairs of points it is a point that is making
      troubles. Therefore, it makes more sense to me to remove a point from
      a data set (and so all the associated pairs) than few pairs. It is the price
      to pay to remain coherent during the analysis.

      Using the pairs more than once, with a �moving lag� can be done to densify the
      number of points of your semivariogram and get so more details. This kind of
      moving windows approach has already been done
      in:

      FLAMM C., KANEVSKY M. & SAVELIEVA E. (1994). Non-regular lag variography and
      multi-method mapping to determination of origin of heavy metals. Case study on
      Geneva heavy metal survey, Switzerland. In: �Proceedings of the Annual
      Conference of the International Association for Mathematical Geology�.
      Mont-Tremblant, Canada, pp. 128-133.

      Hope this helps

      Gregoire

      PS: sorry but I can't find this paper anymore and can�t send it to you

      Rubens Caldeira Monteiro <rubenscm@...> wrote:
      > Dear all,
      >
      > I'm using a MATLAB program to plot covariance (and to find a model, as
      > we usually do for variagrams in Geostatistics).
      >
      > Till today I've just used lag tolerance as half of the spatial lag,
      > larger as possible to take account of all possible distances and smaller
      > as possible to don't take in account the same pairs.
      >
      > This program allow us to define different lag-tolerance to different lags,
      > but doing this the mean of pais covariance for each lag, considering each
      > lag-tolerance, is different, and allow us to choose easilly a model.
      >
      > Is better than do variograms/covariograms for differents lags to see wich
      > lag give us a good experimental variogram/cov., even considering the
      > "physical knowledge" (geology, limits etc.) to choose lags.
      >
      > My question is: Do we have problems with our var./cov. if we don't
      > consider some pairs or take in account the same pairs more than one time
      > (overlaping)? Or we can consider this like a flexibility, depending on the
      > expert knowledge or judgement?
      >
      > Thanks,
      >
      > Rubens Caldeira Monteiro
      > PhD. candidate on "Geosciences and Environmental Sciences"
      >
      > University of the State of Sao Paulo at Rio Claro - UNESP-RC
      > http://www.rc.unesp.br/igce/dga
      > rubenscm@...
      >
      > University of North Carolina at Chapel Hill - UNC-CH (visiting scholar)
      > http://www.unc.edu/~rubenscm
      > rcmonteiro@...
      >
      >
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      Gregoire Dubois
      Institute of Mineralogy and Petrography
      Dept. of Earth Sciences
      University of Lausanne
      Switzerland

      http://www.ai-geostats.org

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