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AI-GEOSTATS: "strength" of autocorrelation

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  • Volker Bahn
    Dear list members, I would like your opinions on my ideas on the definition of strength of autocorrelation as interpreted through a variogram. There seem to
    Message 1 of 2 , Feb 8, 2004
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      Dear list members,

      I would like your opinions on my ideas on the definition of "strength" of
      autocorrelation as interpreted through a variogram. There seem to be two
      important parts: a) the range and b) the shape of the variogram. (Whereas
      the sill and the nugget are in my opinion not of significance to the problem
      as the former is only an expression of the variability in the data and the
      latter should theoretically not exist at all and is most likely a
      measurement error or distance bin artifact.) So for a constant range, the
      more shallow the variogram the stronger the autocorrelation and for a
      constant curve, the longer the range the stronger the autocorrelation. Could
      I maybe capture both notions by extracting the area under (i.e., intergrate
      over) a correlogram (let's say up to where it crosses zero)? Has anyone
      invented or successfully used such a measure? Does it make sense to you?

      Cheers

      Volker

      -~^/\^~^/\~/\^~^^/\/\~^~*

      Dept. of Wildlife Ecology - Rm. 210
      University of Maine
      5755 Nutting Hall
      Orono, Maine
      04469-5755, USA
      Tel: (207) 581 2799
      Fax: (207) 581 2858


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    • Isobel Clark
      Volker I don t understand how you can describe a curve as shallow if you consider the sill to be irrelevant. How do you measure shallowness without a
      Message 2 of 2 , Feb 9, 2004
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        Volker

        I don't understand how you can describe a curve as
        'shallow' if you consider the sill to be irrelevant.
        How do you measure 'shallowness' without a reference
        to the sill?

        I woudl also like to say that your hard line approach
        to the nugget effect is not shared by those of us who
        work at the 'sharp end'. I have investigated cases
        where the nugget effect is clearly visible as part of
        the structure of the phenomenon and have had the
        opportunity to research into controlled trials on
        contiguous sampling on several occasions.

        No matter how close you take your sample, no matter
        how well you sample, there will always be a difference
        between two contiguous samples. This difference may be
        minimal, in which case you will see no nugget effect.
        Sometimes it is a significant part of the total
        variation of the sample values.

        Some of the nugget effect will be errors introduced by
        sampling, but only a full balanced statistical study
        will convince me that all of the nugget effect is
        sampling error.

        Isobel Clark
        http://geocities.bizland.com/whatsnew.htm



        --- Volker Bahn <lochapoka@...> wrote: > Dear list
        members,
        >
        > I would like your opinions on my ideas on the
        > definition of "strength" of
        > autocorrelation as interpreted through a variogram.
        > There seem to be two
        > important parts: a) the range and b) the shape of
        > the variogram. (Whereas
        > the sill and the nugget are in my opinion not of
        > significance to the problem
        > as the former is only an expression of the
        > variability in the data and the
        > latter should theoretically not exist at all and is
        > most likely a
        > measurement error or distance bin artifact.) So for
        > a constant range, the
        > more shallow the variogram the stronger the
        > autocorrelation and for a
        > constant curve, the longer the range the stronger
        > the autocorrelation. Could
        > I maybe capture both notions by extracting the area
        > under (i.e., intergrate
        > over) a correlogram (let's say up to where it
        > crosses zero)? Has anyone
        > invented or successfully used such a measure? Does
        > it make sense to you?
        >
        > Cheers
        >
        > Volker
        >
        > -~^/\^~^/\~/\^~^^/\/\~^~*
        >
        > Dept. of Wildlife Ecology - Rm. 210
        > University of Maine
        > 5755 Nutting Hall
        > Orono, Maine
        > 04469-5755, USA
        > Tel: (207) 581 2799
        > Fax: (207) 581 2858
        >
        >
        > --
        > * 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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