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AI-GEOSTATS: Declustering

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  • amazot@ulb.ac.be
    Hello, I would like to know what is the declustering, why we used this method and how we proceed to decluster a set of data? Thank you -- * To post a message
    Message 1 of 7 , Nov 15, 2001
      Hello,

      I would like to know what is the declustering, why we used this
      method and how we proceed to decluster a set of data?

      Thank you

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    • Alessandro Gimona
      Isobel wrote: .....The semi-variogram and kriging techniques are not affected by clustering. ... I am slightly confused: ... if sampling is clustered
      Message 2 of 7 , Nov 16, 2001
        Isobel wrote:

        .....The semi-variogram and kriging techniques are not affected by
        clustering.
        > In fact, one of the main reasons for inventing kriging
        > was to make full use of every one of clustered and/or
        > preferentially sited sampling.
        >
        >
        I am slightly confused:
        ... if sampling is clustered preferentially in e.g. higher values
        areas, would this not bias the semi-variogram for the first few lags?...at
        least if, as it can happen, the variance is related to the mean.
        What about the effects of the possible over-estimation of the global
        mean due to clustering?

        thanks

        Alessandro Gimona

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      • Isobel Clark
        ... The semi-variogram is calculated on the difference between the two sample values. If the basic assumptions for semi-variogram construction are correct,
        Message 3 of 7 , Nov 16, 2001
          > I am slightly confused:
          > ... if sampling is clustered preferentially in
          > e.g. > higher values
          > areas, would this not bias the semi-variogram for
          > the first few lags?...at
          > least if, as it can happen, the variance is related
          > to the mean.
          The semi-variogram is calculated on the difference
          between the two sample values. If the basic
          assumptions for semi-variogram construction are
          correct, differences are unrelated to the actual value
          of each sample or to the actual absolute location of
          the pair. Therefore, clustering does not influence the
          semi-variogram.

          If you have a situation where variance is related to
          the mean, e.g. with highly skewed data, you need to
          transform these values in some way before constructing
          a semi-variogram. This is true whether or not you have
          clustered sampling. Absolutely regular sampling will
          not give you a valid semi-variogram if you violate the
          assumptions upon which it is based.

          > What about the effects of the possible
          > over-estimation of the global mean due to
          clustering?
          If you are estimating the global mean based on a
          distribution model, you need to decluster. If you are
          estimating the global meaning on the basis of a kriged
          grid, you do not need to decluster as the kriging
          system does that for you.

          You can experiment with these questions using our
          totally free unlimited kriging game. This can be found
          in my 'briefcase' at
          http://uk.geocities.com/drisobelclark/briefcase.html

          Does this help?
          Isobel Clark

          __________________________________________________
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        • oliver.sonnentag@web.de
          hello list, to get the global mean of my data set (484 observation wells) and as a prerequisite for Normal Score Transformation with GSLIB, i declustered my
          Message 4 of 7 , Jul 5, 2003
            hello list,
            to get the global mean of my data set (484 observation wells) and
            as a prerequisite for Normal Score Transformation with GSLIB, i
            declustered my data with polygonal declustering. When
            standardizing the weights to 1, so that the weights sum up to the
            number of data, I receive some negative weights. this results in
            negative values for the observations (nitrate concentration). how do
            i have to interpret this ? does this require a special treatment? skip
            them? as I mentioned before, i want to normal score transform the
            data set prior to simple kriging.
            many many thanx for some help in advance.
            Regards,
            Oliver




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          • Pierre Goovaerts
            Hi Oliver, I don t know which algorithm you are using to compute these declustering weights but there is something wrong in your rescaling procedure. These
            Message 5 of 7 , Jul 5, 2003
              Hi Oliver,

              I don't know which algorithm you are using to compute these declustering
              weights but there is something wrong in your rescaling procedure.
              These declustering weights are proportional to the size of the polygon
              of influence of each observation and they can not be negative.

              Regards,

              Pierre Goovaerts

              <><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><>

              Dr. Pierre Goovaerts
              President of PGeostat, LLC
              Chief Scientist with Biomedware Inc.
              710 Ridgemont Lane
              Ann Arbor, Michigan, 48103-1535, U.S.A.

              E-mail: goovaert@...
              Phone: (734) 668-9900
              Fax: (734) 668-7788
              http://alumni.engin.umich.edu/~goovaert/

              <><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><>

              On Sat, 5 Jul 2003 oliver.sonnentag@... wrote:

              > hello list,
              > to get the global mean of my data set (484 observation wells) and
              > as a prerequisite for Normal Score Transformation with GSLIB, i
              > declustered my data with polygonal declustering. When
              > standardizing the weights to 1, so that the weights sum up to the
              > number of data, I receive some negative weights. this results in
              > negative values for the observations (nitrate concentration). how do
              > i have to interpret this ? does this require a special treatment? skip
              > them? as I mentioned before, i want to normal score transform the
              > data set prior to simple kriging.
              > many many thanx for some help in advance.
              > Regards,
              > Oliver
              >
              >
              >
              >
              > --
              > * 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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              > * Support to the list is provided at http://www.ai-geostats.org
              >


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            • Michael Pyrcz
              Good day Oliver, I have another comment on declustering in addition to the previous comment on negative weights. The weights do not actually change the data
              Message 6 of 7 , Jul 5, 2003
                Good day Oliver,

                I have another comment on declustering in addition to the previous
                comment on negative weights. The weights do not actually change the
                data values. They change the weight of the data values within the
                distribution; therefore, weights change the distribution, but not the
                sample data. One way to think of declustering weights is that they
                would change the height on the bars of a histogram (more precisely, the
                contribution of each of the data to the bar height), but not the
                position of the bars.

                The GSLIB book (Deutsch and Journel) has a lot of practical guides to
                declustering. I have recently written a paper on the specifics of
                declustering, if you are interested.

                All the best,

                Michael

                _____________________________________________

                Michael Pyrcz
                Ph.D. Candidate
                Department of Civil & Environmental Engineering
                University of Alberta
                (780) 492-3153
                mpyrcz@...
                www.ualberta.ca/~mpyrcz


                -----Original Message-----
                From: ai-geostats-list@... [mailto:ai-geostats-list@...] On
                Behalf Of oliver.sonnentag@...
                Sent: July 5, 2003 2:13 AM
                To: ai-geostats@...
                Subject: AI-GEOSTATS: Declustering

                hello list,
                to get the global mean of my data set (484 observation wells) and
                as a prerequisite for Normal Score Transformation with GSLIB, i
                declustered my data with polygonal declustering. When
                standardizing the weights to 1, so that the weights sum up to the
                number of data, I receive some negative weights. this results in
                negative values for the observations (nitrate concentration). how do
                i have to interpret this ? does this require a special treatment? skip
                them? as I mentioned before, i want to normal score transform the
                data set prior to simple kriging.
                many many thanx for some help in advance.
                Regards,
                Oliver




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