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[ai-geostats] XValid script

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  • Mark Dowdall
    Hello It may be cheeky, but I have two questions: 1. Where can I get I get the Surfer v7.0 script for crossvalidation? I have been through the archives and
    Message 1 of 2 , Sep 1, 2004
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      Hello

      It may be cheeky, but I have two questions:

      1. Where can I get I get the Surfer v7.0 script for crossvalidation? I
      have been through the archives and although its mentioned I cannot find
      a download site. Nor could I find it and the site of Aleksey Amantov.
      Some emails mentioned it is in the samples folder of the surfer install
      but I could not find it. I know its in Surfer 8 as standard but thats
      not available to me at the moment.

      2. I have been following the discussion on extreme values in a data set.
      And my question is: is there any context in which geostatistical methods
      are absolutely not appropriate? If so, how can this appropriateness be
      tested? Is there a point at which a user should know that it would be
      better to try something else?

      Any help with these two is very much welcome

      mdowdall
    • Gregoire Dubois
      Hi Mark, 1) At http://www.ai-geostats.org/software/Codes_Softfaq.htm you will find a few codes for Surfer. I have just added crossvalidation.bas , a code I
      Message 2 of 2 , Sep 1, 2004
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        Hi Mark,

        1) At

        http://www.ai-geostats.org/software/Codes_Softfaq.htm

        you will find a few codes for Surfer. I have just added
        crossvalidation.bas , a code I derived from Aleksey Amantov's
        jacknife.bas
        You will need to code a bit yourself to select the correct interpolation
        function (Inverse Distance Weighted is the default) as well as the
        proper parameters. Surfers manual is sufficient to learn to program with
        Scripter (quick basic). Sorry, but I will not provide any support. Note
        that crossvalidations are implemented in Surfer 8.

        2) In short: Geostatistics are not appropriate when you don't have a
        spatial correlation (even this is questionable :) ).
        Frequently, people who do not find any spatial correlation in their data
        apply other functions like Inverse Weighted Distance. I believe such a
        solution is completelt wrong. Either you have a spatial correlation and
        you can interpolate/predict values, or you don't and only models based
        on sound physics (e.g. Temperature T decreases with latitude and
        altitude, thus you can predict T) can be used. Outside of these two
        solutions, I would recommend only to use Thiessen polygons for
        displaying purposes (not for real estimations!): they do not look
        "natural" when applied to continuous variables and can thus not fool the
        persoon looking at the output. Moreover, Thiessen polygons provides
        useful information on the weight of each sample. Maps of proportional
        symbols would also do.

        Cheers,

        Gregoire

        -----Original Message-----
        From: Mark Dowdall [mailto:mark.dowdall@...]
        Sent: 01 September 2004 10:02
        To: ai-geostats@...
        Subject: [ai-geostats] XValid script




        Hello

        It may be cheeky, but I have two questions:

        1. Where can I get I get the Surfer v7.0 script for crossvalidation? I
        have been through the archives and although its mentioned I cannot find
        a download site. Nor could I find it and the site of Aleksey Amantov.
        Some emails mentioned it is in the samples folder of the surfer install
        but I could not find it. I know its in Surfer 8 as standard but thats
        not available to me at the moment.

        2. I have been following the discussion on extreme values in a data set.
        And my question is: is there any context in which geostatistical methods
        are absolutely not appropriate? If so, how can this appropriateness be
        tested? Is there a point at which a user should know that it would be
        better to try something else?

        Any help with these two is very much welcome

        mdowdall
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