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AI-GEOSTATS: Re: Urgent help needed: Power-law Semi-variogram Range

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  • Joseph Oyeyele
    Thank you for your reply, By the transect method, I mean by taking a straight line through the field of observation. The transect method computes the variogram
    Message 1 of 2 , Feb 11, 2004
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      Thank you for your reply,

      By the transect method, I mean by taking a straight line through the field of observation. The transect method computes the variogram along this line only, since it is an isotropic process generated from a functional form.

      Is there a reliable method for determining the range(correlation length).
      I wrote a function in Matlab to determine and plot the variogram. I cross-checked by running the generated data through GSTAT --- it also results in a power law variogram.

      Please advise on the best approach to determine the correlation length in this case.

      Thanks

      Joseph.


      ----- Original Message -----
      From: Gerald Boogaart <boogaart@...>
      Date: Wed, 11 Feb 2004 11:36:20 +0100
      To: seacode@...
      Subject: Re: Urgent help needed: Power-law Semi-variogram Range

      > Dear Joseph,
      >
      > I don't know wether I can help, not knowing what you mean by transect method.
      > Anyway others can give better answers later.
      >
      > As far as I understand the subject there is no range for a power law
      > variogram, since it is unbounded. Anyway some software will ask for a range,
      > which is that either ignored or used in some softwarespecific way e.g. in the
      > way gamma(h)=nugget + sill* (h/range)^p where it is redundent with the sill.
      > Just try to find out, and fit visually or reask the question specifing the
      > software you are using. Anyway when the software is asking for a sill or
      > range in power law variograms the software uses some odd convention.
      >
      > Best regards,
      > Gerald
      >
      > On Tuesday 10 February 2004 21:04, you wrote:
      > > Hi Geostats,
      > >
      > > Kindly advise me on how to determine the range value from a power-law semi
      > > variogram. I am new to geostatistics but I need to analyse sample data from
      > > an isotropic process. I am getting a power-law. How can I determine a range
      > > value? I am using the transect method.
      > >
      > > Thanks in advance!
      > >
      > > Joseph
      >
      >

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    • Edzer J. Pebesma
      Joseph, not all spatial processes have correlation lengths; some processes show an increase in variance whenever you increase the domain. Think of (1D case)
      Message 2 of 2 , Feb 12, 2004
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        Joseph,

        not all spatial processes have correlation lengths; some
        processes show an increase in variance whenever you
        increase the domain. Think of (1D case) Brownian
        motion. Power law variograms result from processes
        that have no correlation length; IIRC, Brownian
        motion has a linear variogram (power=1) with no sill.
        --
        Edzer

        Joseph Oyeyele wrote:

        >Thank you for your reply,
        >
        >By the transect method, I mean by taking a straight line through the field of observation. The transect method computes the variogram along this line only, since it is an isotropic process generated from a functional form.
        >
        >Is there a reliable method for determining the range(correlation length).
        >I wrote a function in Matlab to determine and plot the variogram. I cross-checked by running the generated data through GSTAT --- it also results in a power law variogram.
        >
        >Please advise on the best approach to determine the correlation length in this case.
        >
        >Thanks
        >
        >Joseph.
        >
        >
        >


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