## Re: AI-GEOSTATS: SA using GSLIB

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• Dear Uwe, The interpretation of the nlag parameter in sasim can be somewhat confusing. It corresponds to the number of lags in all directions to be
Message 1 of 2 , Jun 13, 2003
Dear Uwe,

The interpretation of the nlag parameter in sasim
can be somewhat confusing. It corresponds to the number
of lags in all directions to be incorporated in the objective
functions, not the class of distance for the semivariogram
model. For example, to reproduce the variogram for
the 1st "lag distance" of 0.5 km in all directions,
you need to specify nlags=8, which corresponds to
the number of cells in the direct neighborhood of the cell
being simulated. For reproducing the 2nd class of distance
of 1km, nlags should be 24. So the formula to use to
reproduce K classes of distances would be
nlag=[(2K+1)^2]-1. So if you want to reproduce the variogram
up to a distance of 20 kms using a grid size of 0.5km
nlag=[(2x40+1)^2]-1=6560! I have recently looked at ways
to sample this set of lags since it is clearly impossible
to afford such a large number of lags in the objective function.
For example, only a random subset of these 6,560 possible lags
would be reproduced. This is a limitation of the simulated annealing
algorithm (would be even worst in 3D) and any other suggestions
would be much appreciated.

Regards,

Pierre Goovaerts

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

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 Fri, 13 Jun 2003, Uwe Haberlandt wrote:

> Dear all,
>
> I have a question concerning the simulated annealing module within GSLIB
> (f77v2 and f90). I'm using kilometers as units with grid spacing of 0.5
> km and wanted a reproduction of the variogram up to a distance of 20 km.
> The range is 10 km. However for different nlags set in the parameter
> file (e.g. 20, 40, 80,120) the debugging file reports variogram values
> (actual and model) much lower than expected. In the debugging file the
> model variogram does not reach the sill not even with nlags=120. How is
> "nlags" interpreted and what is reported in the debug file?
>
> Any help is greatly appreciated
> Regards
> Uwe
>
>
> **********************************************************
> Dr. Uwe Haberlandt
> Ruhr University Bochum
> Institute for Hydrology, Water Management
> and Environmental Engineering
> Universitätsstraße 150
> 44780 Bochum
> Germany
> Tel.: +49 (0)234-32-27619
> Fax.: +49 (0)234-32-14153
> e-mail: uwe.haberlandt@...
> www: http://www.ruhr-uni-bochum.de/hydrology/
> **********************************************************
>
>

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