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!

Thanks a lot ahead!

Xiuh

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your questions are long-standing and as yet unanswered

in general.

> 1. How to select the lag class and lag distance in

I always think of it as focussing a camera. Believe

> order to obtain a more reasonable experimental

> variogram?

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.

Check out our free tutorial downloads at

http://geoecosse.bizland.com/softwares

> 2. Is it reasonable to use an uneven set of lag

I am not sure I have ever seen this done, but don't

> (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?

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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