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Re: [ai-geostats] Re: about cross-validation

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  • Mahdi Osman
    Raimon Tolosana-Delgado, Many thanks. This was very helpful indeed. Regards Mahdi ... -- ... Mahdi Osman (PhD) E-mail: m_osm@gmx.net ... Echte DSL-Flatrate
    Message 1 of 3 , Apr 27, 2006
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      Raimon Tolosana-Delgado,

      Many thanks. This was very helpful indeed.

      Regards

      Mahdi



      > --- Ursprüngliche Nachricht ---
      > Von: "Raimon Tolosana" <raimon.tolosana@...>
      > An: "Mahdi Osman" <m_osm@...>
      > Kopie: ai-geostats@...
      > Betreff: Re: [ai-geostats] Re: about cross-validation
      > Datum: Thu, 27 Apr 2006 17:10:09 +0200
      >
      > Dear Mahdi
      >
      > Quickly said, you cannot compare both nuggets, because they do not
      > share units... (well, the logarithmic one may be considered to have no
      > units, but this does not change the point).
      >
      > But, why do you want to compare the variograms of your two data sets
      > in different scales? Probably, it would be more logical to take logs
      > of the subset, compute its experimental variogram, model it and
      > afterwards compare both models... always in log scale.
      >
      > However, if you really want to do such comparison, there's a
      > relationship between the variogram for Z (a positive regionalized
      > variable) and Y=ln Z, both second-order stationary:
      > $$
      > \gamma_{Z}(h) = E[Z]^2 \cdot \exp{C_{Y}(0)} \cdot
      > (1-\exp{-\gamma_{Y}(h)} )
      > $$
      > This comes from the relationship between variogram and covariance
      > function ( $C(h)=C(0)-\gamma(h)$ ), and between covariances for Z and
      > Y (which you can find, for instance, in Chilès&Delfiner, 1999, book
      > "Geostatistics: modelling spatial uncertainty"). Pay attention that
      > this is only true if Y is a Gaussian regionalized variable, and if
      > this assumption is false, your variograms may be horribly different.
      >
      > regards
      > Raimon Tolosana-Delgado
      >
      > On 4/27/06, Mahdi Osman <m_osm@...> wrote:
      > > Dear Isobel Clark,
      > >
      > > Dear List,
      > >
      > > I have your posting regrding cross validation. Yesterday and this
      > morning I
      > > had similar, but not identical, problems. So I was wondering if you
      > could
      > > help me with this.
      > >
      > >
      > > I have got two datasets. One of the datasets was a subset of the other.
      > So I
      > > did semivariograms of a variable of interest on both datasets. The major
      > > dataset was logtransformed, whereas the other was not. So now I have
      > got
      > > semivariogram parameters from the logtransformed data and from the
      > > non-transformed data. From the logtransformed data, I have got a smaller
      > > nuget, compared to the nugget from the non-transformed data. My question
      > is:
      > >
      > >
      > > Can I actully compare the two semivariograms directly based on their
      > nuget
      > > values even though one data was transformed and the other was not?
      > >
      > > How can I compare across semivariograms at all, is that possible or not?
      > >
      > > Is it possbile to backtransform the values and then compare the
      > > semivariograms?
      > >
      > >
      > > Thank you very much indeed for your help and time
      > >
      > >
      > > Reagds
      > >
      > >
      > > Mahdi
      > >
      > > --
      > > -----------------------------------
      > > Mahdi Osman (PhD)
      > > E-mail: m_osm@...
      > > -----------------------------------
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      >
      >

      --
      -----------------------------------
      Mahdi Osman (PhD)
      E-mail: m_osm@...
      -----------------------------------

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