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AI-GEOSTATS: global vs local ordinary kriging

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  • Ulrich Leopold
    Dear list, What would you consider the most reliable ordinary kriging estimate? To use a local search neighbourhood (slightly bigger than the effective range)
    Message 1 of 5 , Jul 8, 2003
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      Dear list,

      What would you consider the most reliable ordinary kriging estimate? To
      use a local search neighbourhood (slightly bigger than the effective
      range) or set to global to include *all* data locations?


      Ulrich


      --
      __________________________________________________

      Ulrich Leopold MSc.

      Department of Physical Geography
      Institute for Biodiversity and Ecosystem Dynamics
      Faculty of Science
      University of Amsterdam
      Nieuwe Achtergracht 166
      NL-1018WV Amsterdam

      Phone: +31-(0)20-525-7456 (7451 Secretary)
      Fax: +31-(0)20-525-7431
      Email: uleopold@...
      http://www.frw.uva.nl/soil/Welcome.html

      Check us also out at:
      Netherlands Centre for Geo-ecological Research
      http://www.frw.uva.nl/icg




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    • Isobel Clark
      Ulrich Depends how powerful your computer is, what algorithm you use to solve equations and how many data you have. Isobel
      Message 2 of 5 , Jul 8, 2003
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        Ulrich

        Depends how powerful your computer is, what algorithm
        you use to solve equations and how many data you have.

        Isobel
        http://geoecosse.bizland.com/0toKriging.htm

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      • Pierre Goovaerts
        Hi Ulrich, It s not an easy question. First note that the search strategy includes not only the size of the search window but also the maximum number of
        Message 3 of 5 , Jul 8, 2003
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          Hi Ulrich,

          It's not an easy question. First note that the search strategy
          includes not only the size of the search window but also the maximum
          number of observations. In may occasions, I set the search radius
          to a very large distance and use the number of observations as
          the controling parameter. Using too many observations or too large
          search windows may lead to oversmoothing, while estimates based on
          low number of observations (say less than 8 in 2D) might not be
          very reliable. Of course it depends also on the relative nugget effect.
          If it is large, even further away observations will receive a
          significant weight.

          In practice, global search windows are seldom used because:
          (1) no reliable semivariogram values are available
          for so large distances, (2) the size of the kriging system is
          likely very large, and (3) the stationarity assumption
          within the search window might become questionable.

          The best way to proceed would be to do some cross validation
          using various search strategies and investigate their impact
          on re-estimation scores.

          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 8 Jul 2003, Ulrich Leopold wrote:

          > Dear list,
          >
          > What would you consider the most reliable ordinary kriging estimate? To
          > use a local search neighbourhood (slightly bigger than the effective
          > range) or set to global to include *all* data locations?
          >
          >
          > Ulrich
          >
          >
          > --
          > __________________________________________________
          >
          > Ulrich Leopold MSc.
          >
          > Department of Physical Geography
          > Institute for Biodiversity and Ecosystem Dynamics
          > Faculty of Science
          > University of Amsterdam
          > Nieuwe Achtergracht 166
          > NL-1018WV Amsterdam
          >
          > Phone: +31-(0)20-525-7456 (7451 Secretary)
          > Fax: +31-(0)20-525-7431
          > Email: uleopold@...
          > http://www.frw.uva.nl/soil/Welcome.html
          >
          > Check us also out at:
          > Netherlands Centre for Geo-ecological Research
          > http://www.frw.uva.nl/icg
          >
          >
          >
          >
          > --
          > * 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.
          > * To unsubscribe, send an email to majordomo@... with no subject and "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
          >


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        • Juliann Aukema
          Hi, I have a sampling question. I am planning a study to look at plant diversity and plant distribution patterns in a karstic area. The topography is a series
          Message 4 of 5 , Jul 8, 2003
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            Hi,
            I have a sampling question. I am planning a study to
            look at plant diversity and plant distribution
            patterns in a karstic area. The topography is a series
            of hills and valleys in all directions
            (http://www.cdk-pr.org/index.htm for a picture). I
            have a dataset that contains about 50 sample locations
            distributed through the region (basically a 75
            kilometer transect, 1-15 kilometers wide). At each
            location, plant community data was taken at three
            topographic positions hilltop, hillside, and valley.
            Previous studies have demonstrated a difference in
            plant communities between these three positions,
            therefore I don�t think that this dataset can be
            considered to have 150 points. Seven pairs of points
            are separated by between 45 and 1000 meters, the rest
            are separated by greater distances. I would like to
            expand the sampling so that I have enough points for a
            spatial analysis and to encompass a finer scale (for
            example, neighboring hills). One possibility I am
            considering is taking say 10 evenly distributed
            sampled points and sampling hill, slope, valley along
            a transect for 5 additional hills. My questions are:
            do I need more points for a reasonable spatial
            analysis? how many points would you all recommend �
            minimum number, ideal number? and do you have any
            sampling suggestions other than my transect idea for
            encompassing the finer scale?

            Thanks a lot for suggestions,

            Juliann
            aukemaj@...


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