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 a : ai-geostats@unil.ch 28 April 2006 • 2:49PM -0400

Many thanks. This was very helpful indeed.

Regards

Mahdi

> --- Ursprüngliche Nachricht ---
> Von: "Raimon Tolosana" <raimon.tolosana@gmai...>
> An: "Mahdi Osman" <m_osm@gmx....>
> Kopie: ai-geostats@unil...
> 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
>
> On 4/27/06, Mahdi Osman <m_osm@gmx....> 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@gmx....
> > -----------------------------------
> >
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> >
>
>

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

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