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Re: AI-GEOSTATS: Search options of Kriging

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  • Felus A Yaron
    Hi Muna, Good question. The answer depends on the type of semi-variogram that you have (especially the range - if it has). I feel that the following quotation
    Message 1 of 4 , Jun 1, 2001
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      Hi Muna,

      Good question. The answer depends on the type of semi-variogram
      that you have (especially the range - if it has). I feel that the following
      quotation represents the current knowledge about this topic:
      " .. the neighborhood must be large enough to contain three
      observations at a bare minimum and anything beyond 25 observation
      is considered more then adequate... More precise justification for
      selection of neighborhood size depend upon the measure of
      performance achieved and fluctuates according to the nature
      of the sampling pattern and the covariance. Lacking theoretical criteria,
      experiments ... is the best alternative."
      From: Olea, R.A., 1999. Geostatistics for engineers and earth
      scientists. Kluwer, Boston, Page 63.
      See also:
      Cressie Noel (1993) Statistics for spatial data. pp. 158-160, for some
      practical suggestion, or Chiles and Delfiner (1999) Geostatistics, Modeling
      Spatial Uncertainty. pp. 201- 209 for a complete discussion.

      Good luck,
      Yaron Felus.
      Ohio-State University.

      ----- Original Message -----
      From: Muna Mirghani <mirg0933@...-berlin.de>
      To: <ai-geostats@...>
      Sent: Thursday, May 31, 2001 9:31 PM
      Subject: AI-GEOSTATS: Search options of Kriging


      > Dear list members,
      > I get totally different distributions when I use a "Search" option in
      > kriging compared to a "No-search" option. The data (90 points) I use are
      > not evenly distributed with spacing ranging from 300m to 6000m; a case in
      > which it is recommended to use quadrant or octant search type to avoid
      > search in one direction. However, the resulting map of the no-search is
      > smoother.
      > Please kindly advice me on the criteria to decide on the suitable option.
      > With many Thanks.
      > Muna
      >
      >



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    • Isobel Clark
      ... What is a no search option? Minimum and maximum numbers of samples in kriging depend heavily on what sort of semi-variogram you have and how much nugget
      Message 2 of 4 , Jun 1, 2001
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        > I get totally different distributions when I use a
        > "Search" option in kriging compared to a "No-search"
        > option.
        What is a "no search" option?

        Minimum and maximum numbers of samples in kriging
        depend heavily on what sort of semi-variogram you have
        and how much nugget effect is present. In addition,
        specifying a number rather than (say) a quadrant
        search leaves you with the possibility still of very
        uneven clustered sets of samples used in the
        estimation. Kriging deals with clusters but reasonably
        even coverage is still more efficient.

        Isobel Clark

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      • Colin Daly
        Muna, If the no search algorithm means that all the 90 data are being used then this result (sometimes called kriging with a unique neighbourhood) always
        Message 3 of 4 , Dec 20, 2001
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          Muna,

          If the 'no search' algorithm means that all the 90 data are being used then
          this result (sometimes called kriging with a unique neighbourhood) always
          gives smoother results than using a local neighbourhood. The reason is that
          in local kriging you will krige a part of your field using some
          neighbourhood set but other parts of the field will use different
          neighbours.You tend to see discontinuity as you go from one sector to the
          next (as one neighbour point gets replaced by another). This effect is more
          noticeable when you use small neighbourhood sets. It is reduced for larger
          sets as the points getting swapped out are the distant points and will
          usually have small weights (dependent on the variogram used - they may not
          be small for something like a gaussian variogram for example). Fianlly this
          effect is not there at all if you use all the data points in your
          neighbourhood. So with only 90 points- you should use a unique
          neighbourhood. If the program is well written then you will only ever have
          to solve the kriging equations once and will never have to do any more
          neighbour searchs -so it should run fairly fast. This can be made to run
          faster if there is a dual kriging option in your kriging program - although
          you will not get kriging variance values in this case.

          I can't be sure what 'no search' means - you will have to look it up in the
          software manual - but my guess is that it refers to unique neighbourhood if
          you are seeing smoother results.

          Colin Daly


          ----- Original Message ----
          From: "Isobel Clark" <drisobelclark@...>
          To: "Muna Mirghani" <mirg0933@...-berlin.de>
          Cc: <ai-geostats@...>
          Sent: Friday, June 01, 2001 9:51 AM
          Subject: Re: AI-GEOSTATS: Search options of Kriging


          > > I get totally different distributions when I use a
          > > "Search" option in kriging compared to a "No-search"
          > > option.
          > What is a "no search" option?
          >
          > Minimum and maximum numbers of samples in kriging
          > depend heavily on what sort of semi-variogram you have
          > and how much nugget effect is present. In addition,
          > specifying a number rather than (say) a quadrant
          > search leaves you with the possibility still of very
          > uneven clustered sets of samples used in the
          > estimation. Kriging deals with clusters but reasonably
          > even coverage is still more efficient.
          >
          > Isobel Clark
          >
          > ____________________________________________________________
          > Do You Yahoo!?
          > Get your free @... address at http://mail.yahoo.co.uk
          > or your free @... address at http://mail.yahoo.ie
          >
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