## [ai-geostats] A question on lag class and lag distance

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• Dear all, I have a question on the selection of the lag classes and lag distance when computing experimental semi-variogram. It has been suggested that the
Message 1 of 2 , Oct 1, 2004

Dear all,

I have a question on the selection of the lag classes and lag distance when computing experimental semi-variogram. It has been suggested that the average semivariance can be done in an increment with an arbitrary distance tolerance (e.g. 300 m ± 150 m) taking into consideration of the number of data pairs per lag class. However, the lag class and lag distance are set a bit arbitrarily, and can influence the resulting variogram. My question is:

1. How to select the lag class and lag distance in order to obtain a more reasonable experimental variogram?

2. Is it reasonable to use an uneven set of lag (e.g. the lag increments are: 0-2.5m, 2.5-5.0m, 5.0-12.0m, 12.0-19.5m, 19.5-27.0m, 27.0-30.0m, 30.0-40m, 40-50m etc.) if a more stable variogram can be obtained?

I should really appreciate anyone’s reply!

Xiuh

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• xhy your questions are long-standing and as yet unanswered in general. ... I always think of it as focussing a camera. Believe there is a pattern in your data
Message 2 of 2 , Oct 1, 2004
xhy

in general.

> 1. How to select the lag class and lag distance in
> order to obtain a more reasonable experimental
> variogram?
I always think of it as focussing a camera. Believe
there is a pattern in your data and our task is to
balance 'width of interval' versus 'number of pairs in
interval' to get the clearest picture.

One of the things I have found most useful with
irregularly spaced data is a 'nearest neighbour'
analysis. Take each sample and find the closest one to
it. Record the distance. Repeat for all samples. This
process takes twice as long as calculating the
semi-variogram but gives you an idea of the 'natural'
or model spacing between your samples. This can be
used to guide your choice of interval.

http://geoecosse.bizland.com/softwares

> 2. Is it reasonable to use an uneven set of lag
> (e.g. the lag increments are: 0-2.5m, 2.5-5.0m,
> 5.0-12.0m, 12.0-19.5m, 19.5-27.0m, 27.0-30.0m,
> 30.0-40m, 40-50m etc.) if a more stable variogram
> can be obtained?
I am not sure I have ever seen this done, but don't
see why not if you plot the point at the centre of
gravity of your interval (i.e. average distance of
pairs found).

Hope this helps
Isobel
http://geoecosse.bizland.com/books.htm

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