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RE: [ai-geostats] Understanding variograms

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  • Dan Bebber
    It probably means that you have a spatial trend in your data. Remove any trend, then try again. Dan Bebber Department of Plant Sciences University of Oxford
    Message 1 of 4 , Nov 3, 2004
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      It probably means that you have a spatial trend in your data.
      Remove any trend, then try again.

      Dan Bebber

      Department of Plant Sciences
      University of Oxford
      South Parks Road
      Oxford OX1 3RB
      UK
      Tel. 01865 275000



      > -----Original Message-----
      > From: Jose Luis Gomez Dans [mailto:jgomezdans@...]
      > Sent: 03 November 2004 16:19
      > To: ai-geostats@...
      > Subject: [ai-geostats] Understanding variograms
      >
      >
      > Hi all,
      > I'm new to the list, so bear with me :)
      > I'm trying to carry out some kriging of some (x,y,v)
      > data set on to a
      > regular grid. I have read some of the literature, and
      > I think that
      > this would be a good option (I have also examined the possibility of
      > interpolating using a TIN, but I feel a bit more
      > "safe" using a
      > statistical approach!).
      >
      > In order to do my kriging, I need to have a model
      > semi-variogram, and I
      > thought about using gstat for this (from the command
      > line, no R
      > interfaces, my files are just ASCII (x,y,z) triplets).
      > Given that my
      > data files are huge (of the order of millions of
      > points), I selected a
      > region of interest, and tried to produce a sample
      > semi-variogram, with
      > views to fitting a model.
      >
      > In gstat, I got something that resembles y=exp(x),
      > which looks
      > distinctly wrong. The semivariogram value is ever
      > increasing, and does
      > not reach a sill at all. This is strange, and rather
      > unexpected. I have
      > tried doing a log transform of the data, but the shape
      > of the curve is
      > mostly unchanged.
      >
      > I would be very grateful if anyone could shed any
      > light on what this
      > variogram means, how I could make progress modelling
      > it and so on.
      >
      > Many thanks!
      > José
      >
      > --
      > Jose L Gomez-Dans, Research Assistant
      > Bristol Glaciology Centre, Geographical Sciences/CPOM
      > University of Bristol, Bristol, UK
      >
      >
      >
      >
      >
      >
      > ___________________________________________________________ALL
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    • Dan Bebber
      Detrending is a pretty basic practice in geostatistics. I think you should do some more reading before you plunge into analyses. Dan Bebber ...
      Message 2 of 4 , Nov 3, 2004
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        Detrending is a pretty basic practice in geostatistics. I think you should
        do some more reading before you plunge into analyses.

        Dan Bebber

        > -----Original Message-----
        > From: Jose Luis Gomez Dans [mailto:jgomezdans@...]
        > Sent: 03 November 2004 17:01
        > To: Dan Bebber
        > Subject: Re: [ai-geostats] Understanding variograms
        >
        >
        > Dan and Isobel,
        > Many thanks for your prompt reply!
        >
        > On Wednesday 03 Nov 2004 16:39, you wrote:
        > > It probably means that you have a spatial trend in
        > your data.
        > > Remove any trend, then try again.
        >
        > OK, my data are point heights above the geoid over a
        > large area. While
        > there could well be a trend, how would I go
        > de-trending the data? I
        > have a digital elevation model of this region, and I
        > guess I could
        > subtract the grid value for each point considered, and
        > that should get
        > rid of things like slope effects (in effect, a
        > linear-ish trend).
        >
        > Does this make any sense?
        >
        > Many thanks
        > Jose
        >
        > --
        > Jose L Gomez-Dans, Research Assistant
        > Bristol Glaciology Centre, Geographical Sciences/CPOM
        > University of Bristol, Bristol, UK
        >
        >
        >
        >
        >
        >
        > ___________________________________________________________ALL-NEW
        > Yahoo! Messenger - all new features - even more fun!
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      • Pierre Goovaerts
        Another explanation for this shape is that Jose is looking at the variogram for short distances. It is well known that for very continuous attributes, such as
        Message 3 of 4 , Nov 3, 2004
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          Another explanation for this shape is that Jose is looking at
          the variogram for short distances. It is well known that for
          very continuous attributes, such as elevation or depth to water table.
          the semivariogram is expected to display a parabolic behaviour at
          the origin, which can be modeled using a Gaussian or cubic model
          for example. If one computes the semivariogram before it reaches
          its sill, then one will look only at a power curve and conclude
          that a trend is present. As always, everything is a matter of scale,
          and what looks like as a trend at a local scale can be modeled as
          part of a stationary process at a more regional scale.

          Cheer,

          Pierre
          <><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><>

          Dr. Pierre Goovaerts
          President of PGeostat, LLC
          Chief Scientist with Biomedware Inc.
          710 Ridgemont Lane
          Ann Arbor, Michigan, 48103-1535, U.S.A.

          E-mail: goovaert@...
          Phone: (734) 668-9900
          Fax: (734) 668-7788
          http://alumni.engin.umich.edu/~goovaert/

          <><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><>

          On Wed, 3 Nov 2004, Dan Bebber wrote:

          > Detrending is a pretty basic practice in geostatistics. I think you should
          > do some more reading before you plunge into analyses.
          >
          > Dan Bebber
          >
          > > -----Original Message-----
          > > From: Jose Luis Gomez Dans [mailto:jgomezdans@...]
          > > Sent: 03 November 2004 17:01
          > > To: Dan Bebber
          > > Subject: Re: [ai-geostats] Understanding variograms
          > >
          > >
          > > Dan and Isobel,
          > > Many thanks for your prompt reply!
          > >
          > > On Wednesday 03 Nov 2004 16:39, you wrote:
          > > > It probably means that you have a spatial trend in
          > > your data.
          > > > Remove any trend, then try again.
          > >
          > > OK, my data are point heights above the geoid over a
          > > large area. While
          > > there could well be a trend, how would I go
          > > de-trending the data? I
          > > have a digital elevation model of this region, and I
          > > guess I could
          > > subtract the grid value for each point considered, and
          > > that should get
          > > rid of things like slope effects (in effect, a
          > > linear-ish trend).
          > >
          > > Does this make any sense?
          > >
          > > Many thanks
          > > Jose
          > >
          > > --
          > > Jose L Gomez-Dans, Research Assistant
          > > Bristol Glaciology Centre, Geographical Sciences/CPOM
          > > University of Bristol, Bristol, UK
          > >
          > >
          > >
          > >
          > >
          > >
          > > ___________________________________________________________ALL-NEW
          > > Yahoo! Messenger - all new features - even more fun!
          > http://uk.messenger.yahoo.com
          >
          >
          >
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