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AI-GEOSTATS: Universal Kriging in ArcInfo

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  • Jason Sawle
    Hello This is my first time on the list and I hope this question is going to the right place. My appologies if not. I am using Arc/Info 7.2.1 on an NT
    Message 1 of 4 , Jan 16, 2001
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      Hello

      This is my first time on the list and I hope this question is going to
      the right place. My appologies if not.

      I am using Arc/Info 7.2.1 on an NT workstation.
      With GRID I am using linear universal kriging to produce maps of heavy
      metal concentration from 127 sample points.

      Could anyone tell me what c0 and b mean. These appear at the start of
      the interpolation proceedure. I have values of 0 and 11580.591
      respectively and I am unsure of what they imply about this method.

      Thanks for your time with this.

      Jason Sawle

      ###############################################################
      Department of Geography
      Canterbury Christ Church University College
      North Holmes Road,
      Canterbury, Tel: 01227 782337
      Kent, Fax: 01227 767531
      CT1 1QU, email: j.sawle57@...
      United Kingdom http://www.cant.ac.uk


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    • Isobel Clark
      ... c0 is standard notation for the nugget effect or small scale random (unpredictable) component. Some schools interpret this as sampling error. In mineral
      Message 2 of 4 , Jan 16, 2001
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        > Could anyone tell me what c0 and b mean. These
        > appear at the start of
        > the interpolation proceedure. I have values of 0 and
        > 11580.591
        > respectively and I am unsure of what they imply
        > about this method.
        c0 is standard notation for the 'nugget effect' or
        small scale random (unpredictable) component. Some
        schools interpret this as sampling error. In mineral
        concentration it is usually a feature of the
        mineralisation. No matter how close you sample, the
        difference between neighbouring samples will always be
        non-zero. You nugget effect of zero implies a smooth
        continuous surface with no random component.

        There is no standard notation for the slope of the
        line, but I would guess that is what b is. In that
        case your b is the amount by which the squared
        difference increases for every unit distance. If you
        have a zero nugget effect, the slope is irrelevant
        except for calculating the standard error of the
        estimation.

        I am not familiar with the details of ArcInfo. Does it
        calculate and fit the semi-variogram model
        automatically? I would be very suspicious of a zero
        nugget effect in an application such as you describe.

        If I can be of any further help, please let me know.
        Isobel Clark
        Alloa, Scotland
        http://uk.geocities.com/drisobelclark

        ____________________________________________________________
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      • Martin Roseveare
        Hi, I would be very uncertain about the validity of the zero nugget in your case as sampled variables such as you describe seem to be almost guaranteed to have
        Message 3 of 4 , Jan 16, 2001
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          Hi,

          I would be very uncertain about the validity of the zero nugget in
          your case as sampled variables such as you describe seem to be
          almost guaranteed to have some nugget from small-scale
          environmental differences alone. Do you have the option of
          specifying a variogram in ArcInfo? Are your samples evenly
          distributed throughout the area you are kriging?

          Martin Roseveare

          > Hello
          >
          > This is my first time on the list and I hope this question is going to the
          > right place. My appologies if not.
          >
          > I am using Arc/Info 7.2.1 on an NT workstation.
          > With GRID I am using linear universal kriging to produce maps of heavy
          > metal concentration from 127 sample points.
          >
          > Could anyone tell me what c0 and b mean. These appear at the start of the
          > interpolation proceedure. I have values of 0 and 11580.591 respectively
          > and I am unsure of what they imply about this method.
          >
          > Thanks for your time with this.
          >
          > Jason Sawle
          >
          > ###############################################################
          > Department of Geography
          > Canterbury Christ Church University College
          > North Holmes Road,
          > Canterbury, Tel: 01227 782337
          > Kent, Fax: 01227 767531
          > CT1 1QU, email: j.sawle57@...
          > United Kingdom http://www.cant.ac.uk


          ________________________________
          Martin Roseveare
          ArchaeoPhysica Ltd.
          Phone: 07050 369789
          Fax: 07050 369790
          email: mail@...
          web: http://www.archaeophysica.co.uk

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        • Patrick van Laake
          Dear listmembers, I tried to send this before, but I don t think anybody but Jason has seen it. If you have, sorry for the inconvenience. (List Manager, maybe
          Message 4 of 4 , Jan 17, 2001
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            Dear listmembers, I tried to send this before, but I don't think anybody but Jason has seen it. If you have, sorry for the inconvenience. (List Manager, maybe it is possible to put ai-geostats@... first and
            then the author such that a simple reply to a post works?)

            >Hello
            >
            >This is my first time on the list and I hope this question is going to
            >the right place. My appologies if not.
            >
            >I am using Arc/Info 7.2.1 on an NT workstation.
            >With GRID I am using linear universal kriging to produce maps of heavy
            >metal concentration from 127 sample points.

            In response to the replies of Isobel Clark and Martin Rosevaere and as a
            general warning to yourself: Yes, ArcInfo does automatically produce a
            semi-variogram and you can specify its functional form (the {method} optional
            parameter). Beware, however, of the method that is applied to parameterize the
            variogram. As is stated in the help file, the Levenberg-Marquardt (LM)
            algorithm is applied. LM is a non-linear least squares method of function
            estimation from sampled data. It works by minimizing chi square over the
            sampled data >>>> using the standard deviations of the individual samples
            <<<<. ArcInfo kriging does not ask for, therefore does not know of nor use the
            standard deviation of the sample data (your point cover). Instead, and this
            completely invalidates ArcInfo kriging IMHO, you specify either a number of
            neighbouring sample points or a radius from which to pick sample points to
            calculate local sample standard deviation. THIS IS COMPLETELY AND ABSOLUTELY
            WRONG!!!!! It undermines the essence of kriging. Do not use kriging in ArcInfo
            if you are trying to do serious work, or if your sample data is of high
            quality. Use a proper tool instead.

            And to conclude an excerpt from ArcInfo help on kriging:

            [quote]
            Kriging is a complex procedure that requires greater knowledge about spatial
            statistics than can be conveyed in this command reference. Before using the
            KRIGING command, you should have a thorough understanding of the fundamentals
            of kriging and have assessed the appropriateness of your data for modeling
            with this technique. If you do not have a good understanding of this
            procedure, it is strongly recommended that you review some of the references
            listed at the end of this command reference.
            [unquote]

            If you have a good understanding of kriging and KRIGING, don't use ArcInfo!

            Patrick van Laake
            ESRI Certified ArcInfo Instructor (really but ;-) )



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