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AI-GEOSTATS: Choosing Lag Distance and Angular Tolerance

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  • Yadollah Waghei
    Hello dears I have a spatial data set contaning n=262 observarion (The variable of interest is Rate of Tuberculosis in 262 counties of Iran). I want to fit
    Message 1 of 5 , May 15, 2001
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      Hello dears
      I have a spatial data set contaning n=262 observarion (The variable of interest is Rate of Tuberculosis in 262 counties of Iran). I want to fit some models to Directional semi-variograms,and then build anisotropic semi-variogram.
      Then questions are
      - Is there any rule for choosing Lag Distance and Angular Tolerance?
      -Also,how we can balance between Lag Distance and Angular Tolerance?
      -Do you agree that both must be very small,as possible?(Such that number of pairs in each lag>20, for example)

      Thank you
      Yadollah Waghei
      Dep.of Biostatistics
      Tarbiat Modarres Univ.(Tehran)Po.Box: 14115-111
      Tel:8011001-3872 Fax:8007989
      ___________________________________________________________________________
      Visit http://www.visto.com/info, your free web-based communications center.
      Visto.com. Life on the Dot.


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    • Isobel Clark
      Think of it as focussing a camera. You are looking for the clearest picture. There will be a certain lag (in each direction) at which the calculation will
      Message 2 of 5 , May 15, 2001
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        Think of it as focussing a camera. You are looking for
        the clearest picture. There will be a certain lag (in
        each direction) at which the calculation will balance
        between detail on distance, detail on angles and
        number of pairs.

        In general:

        # the more anisotropic the structure, the smaller the
        tolerance on angle should be

        # a natural interval can be found from the 'nearest
        neighbour' distances between sample locations

        # you won't hit it on the first shot!

        Isobel Clark
        http://uk.geocities.com/drisobelclark

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      • Andrew Mullens
        I have a question relating to this question, certainly not to question the previous writer, it just seems like a good time to bring it up. Will variograming
        Message 3 of 5 , May 15, 2001
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          I have a question relating to this question, certainly not to question the
          previous writer, it just seems like a good time to bring it up. Will
          variograming and other such techniques work for the data the previous writer
          described, e.g samples aren't at points, but areas (and areas that might
          have very little to do with the question). If they did use points in the
          calculations where would the points be placed, at the center of the county,
          at the major population center, at some arbitrary point (e.g most northerly
          point).

          I may be miss reading the description, perhaps the sample are point samples,
          but were taken with one sample in each county.

          Obviously the point samples are never really point sample, they must be
          taken over some area, approximating a point, but does this design seem to
          push the boundaries on that assumption.

          Andrew
          ----- Original Message -----
          From: "Yadollah Waghei" <waghei@...>
          To: <ai-geostats@...>
          Cc: <waghei_y.MED.TMU@...>;
          <mohsen_m@...>; <waghei@...>;
          <mohsen_m@...>; <kazem_an@...>; <Faghihzadeh@...>
          Sent: Tuesday, May 15, 2001 7:33 AM
          Subject: AI-GEOSTATS: Choosing Lag Distance and Angular Tolerance


          > Hello dears
          > I have a spatial data set contaning n=262 observarion (The variable of
          interest is Rate of Tuberculosis in 262 counties of Iran). I want to fit
          some models to Directional semi-variograms,and then build anisotropic
          semi-variogram.
          > Then questions are
          > - Is there any rule for choosing Lag Distance and Angular Tolerance?
          > -Also,how we can balance between Lag Distance and Angular Tolerance?
          > -Do you agree that both must be very small,as possible?(Such that number
          of pairs in each lag>20, for example)
          >
          > Thank you
          > Yadollah Waghei
          > Dep.of Biostatistics
          > Tarbiat Modarres Univ.(Tehran)Po.Box: 14115-111
          > Tel:8011001-3872 Fax:8007989
          >
          ___________________________________________________________________________
          > Visit http://www.visto.com/info, your free web-based communications
          center.
          > Visto.com. Life on the Dot.
          >
          >
          > --
          > * 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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        • Isobel Clark
          Andrew You can apply standard geostatistics if the measurements are the average (or some similar feature) over an area. It makes interpeting the
          Message 4 of 5 , May 15, 2001
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            Andrew

            You can apply 'standard' geostatistics if the
            measurements are the 'average' (or some similar
            feature) over an area.

            It makes interpeting the semi-variogram extremely
            tricky if you combine many different sizes of sample,
            but common sense is the main thing here. The trick is
            to derive a point semi-variogram model from which any
            size can be derived (see Chapter 3, Practical
            Geostatistics 1979, downloadable free from Web,
            http://uk.geocities.com/drisobelclark/practica.html)

            Kriging is modified to reflect that the samples are
            averages, mainly by changing the diagonal elements in
            the equations so that they are non-zero. I don't know
            any software package (off hand ) that does this,
            though.

            Isobel Clark



            --- Andrew Mullens <andrew_mullens@...>
            wrote: > I have a question relating to this question,
            > certainly not to question the
            > previous writer, it just seems like a good time to
            > bring it up. Will
            > variograming and other such techniques work for the
            > data the previous writer
            > described, e.g samples aren't at points, but areas
            > (and areas that might
            > have very little to do with the question). If they
            > did use points in the
            > calculations where would the points be placed, at
            > the center of the county,
            > at the major population center, at some arbitrary
            > point (e.g most northerly
            > point).
            >
            > I may be miss reading the description, perhaps the
            > sample are point samples,
            > but were taken with one sample in each county.
            >
            > Obviously the point samples are never really point
            > sample, they must be
            > taken over some area, approximating a point, but
            > does this design seem to
            > push the boundaries on that assumption.
            >
            > Andrew
            > ----- Original Message -----
            > From: "Yadollah Waghei" <waghei@...>
            > To: <ai-geostats@...>
            > Cc: <waghei_y.MED.TMU@...>;
            > <mohsen_m@...>; <waghei@...>;
            > <mohsen_m@...>; <kazem_an@...>;
            > <Faghihzadeh@...>
            > Sent: Tuesday, May 15, 2001 7:33 AM
            > Subject: AI-GEOSTATS: Choosing Lag Distance and
            > Angular Tolerance
            >
            >
            > > Hello dears
            > > I have a spatial data set contaning n=262
            > observarion (The variable of
            > interest is Rate of Tuberculosis in 262 counties of
            > Iran). I want to fit
            > some models to Directional semi-variograms,and then
            > build anisotropic
            > semi-variogram.
            > > Then questions are
            > > - Is there any rule for choosing Lag Distance and
            > Angular Tolerance?
            > > -Also,how we can balance between Lag Distance and
            > Angular Tolerance?
            > > -Do you agree that both must be very small,as
            > possible?(Such that number
            > of pairs in each lag>20, for example)
            > >
            > > Thank you
            > > Yadollah Waghei
            > > Dep.of Biostatistics
            > > Tarbiat Modarres Univ.(Tehran)Po.Box: 14115-111
            > > Tel:8011001-3872 Fax:8007989
            > >
            >
            ___________________________________________________________________________
            > > Visit http://www.visto.com/info, your free
            > web-based communications
            > center.
            > > Visto.com. Life on the Dot.
            > >
            > >
            > > --
            > > * 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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            > ai-geostats@...
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            > to post a summary of any useful responses to your
            > questions.
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            > followed by "end" on the next line in the message
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            > the list
            > * Support to the list is provided at
            http://www.ai-geostats.org


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          • Isobel Clark
            ... wrote: ... This is something I learned about in quantitative ecology in the 1960 s ;-) You take each sample in turn and find the sample closest to it.
            Message 5 of 5 , May 15, 2001
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              --- Lucie Vallée <Lucie_Vallee@...>
              wrote: >
              >
              > Could you explain what you mean by "nearest
              > neighbour distances between samples locations"?
              This is something I learned about in quantitative
              ecology in the 1960's ;-)

              You take each sample in turn and find the sample
              closest to it. Note the distance. A histogram of the
              distances will give you a sort of natural inter-sample
              distance for use in the semi-variogram calculation.

              The histogram can also be used to check whether
              sampling is randon, uniform, clustered etc.

              I don't have a full reference but it should be easy to
              find: Pielou, Cressie or any other book with Spatial
              Statistics in the title.

              Isobel Clark


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