Hi guys,
I promised myself I would not waste more time
on this futile discussion about covariance and variogram,
but it seems that the discussion has drifted far away from
the initial comment by Isobel or that most people don't
remember what was the initial question.
Isobel's comment originated from my sideline remark
(it was not even part of Celia's initial question)
that the SIMPLE kriging system can not be written in terms of
semivariograms, which Isobel qualified of pure non sense..
It seems that my reference to the excellent
book by Chiles and Delfiner did not convince Isobel.
Let's then use Gslib book by Deutsch and Journel since
it is probably more widely used by members of this discussion
list and Isobel pointed out that anecdote with Andre.
On page 65 of Gslib user manual, 2nd paragraph, I quote:
"In the sytem (IV.4) (SIMPLE kriging system!), the covariance
values C(h) cannot be replaced by semivariogram values
g(h)=C(O)-C(h) unless sum_lambda = 1, which is the ordinary
kriging constraint". I guess it's clear enough, and that is
nothing to do with whether we should solve an ordinary
kriging system in terms of covariances or semivariograms
(Everybody knows that you get the same results!), or
whether we should teach students in one way or another...
Given that SIMPLE kriging is rarely used, we might even
argue that all this discussion is pointless...
Again, the reason for that e-mail is to clarify the matter
for students or practitioners who might have been
confused by this exchange of e-mails... I don't have
a book, a software or a consulting company to advertise!
Cheers,
Pierre
<><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><>
________ ________
| \ / | Pierre Goovaerts
|_ \ / _| Assistant professor
__|________\/________|__ Dept of Civil & Environmental Engineering
| | The University of Michigan
| M I C H I G A N | EWRE Building, Room 117
|________________________| Ann Arbor, Michigan, 48109-2125, U.S.A
_| |_\ /_| |_
| |\ /| | E-mail: [EMAIL PROTECTED]
|________| \/ |________| Phone: (734) 936-0141
Fax: (734) 763-2275
http://www-personal.engin.umich.edu/~goovaert/
<><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><>
On Wed, 23 May 2001, Steve Zoraster wrote:
>
> 1) What manager in the mining or petroleum industry who has graduated
> from college hasn't taken a serious statistics course, including covariances
> and correlations?
>
> 2) Surely when starting from scratch, educating someone about
> geostatistics is more intuitive using covariances? (Just my opinion so far,
> speaking as a mathematician who remembers teaching basic college level
> statistics to nursing majors, education majors, sociology majors, etc. And
> even succeeding occasionally.)
>
> 3) I have taken two multi-day courses in geostatistics from well known
> industry experts. In each class they included significant material and time
> on the first day explaining/justifying variograms by showing their
> mathematical relationship to spatial covariance functions. It seems that
> those instructors did not trust the variogram to be more intuitive than
> spatial covariance functions.
>
> 4) The two basic level introductions to geostatistics I have on my
> bookshelf replicate the experience at those two classes....
>
> Steven Zoraster
>
>
> -----Original Message-----
> From: Yetta Jager [SMTP:[EMAIL PROTECTED]] <mailto:[SMTP:[EMAIL PROTECTED]]>
>
> I think part of the difficulty in the semivariogram vs. covariance war is
> that modeling is subjective, and the notion of covariance has become more
> intuitive for statisticians, while the notion of semivariance has become
> more intuitive for geologists.
>
> From: Isobel Clark [[EMAIL PROTECTED]]
>
> I agree that the semi-variogram approach is easier for the non-statistician
> to grasp. Difference in value is a simpler concept to grasp than
> cross-product, especially when your boss wants to know the likely difference
> between what you tell him and what really happens!
>
>
>
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