## AI-GEOSTATS: Lag - Distance, Variogram / Semivariogram

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• Hi there, can anybody here tell me how to choose the correct (appropriate) lag distance for a regular sampling grid for orchids of 5.5 x 20 metres (Counting
Message 1 of 2 , Sep 4, 2003
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Hi there,

can anybody here tell me how to choose the correct (appropriate) lag
distance for a regular sampling grid for orchids of 5.5 x 20 metres
(Counting data at a certain quadrate) ? I have problems in variogrphy,
because lag distances of 5 metres seem to be problematic because of the
regularity of the sampling grid and the missing values inbetween the 20
meteres distances. If I choose a 15 metres lag I get a much smoother
variogram. Choosing a 5 metres lag, the variance values are too high in the
frist 3 -4 lags.

My second question is what is the difference between a variogram and a
semivariogram ? Is there any ? Is a correlogram easier to interprete ? I
found it much easier to deal with values of my diagram between 0 and 1
rather than having trouble to interprete a particular variance x !!??

Hopefully you can help a newbie.

Ursula:-)

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• Ursula ... This is possibly because you have some competition effect between orchids. For a square grid, we usually recommend an interval 20% of the grid
Message 2 of 2 , Sep 4, 2003
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Ursula

> 5.5 x 20 metres
> Choosing a 5 metres lag, the variance
> values are too high in the first 3 -4 lags.
This is possibly because you have some competition
effect between orchids.

For a square grid, we usually recommend an interval
20% of the grid spacing, so you don't get diagonals
lumped in with 'straight' directions. This would
suggest that you should try a 1 metre lag, which is
probably overkill.

The other alternative is to construct directional
semi-variograms and specify the correct lag for each
direction, to see what differences you get.

> My second question is what is the difference between
> a variogram and a semivariogram ?
As a general rule a "variogram" is a semi-variogram
constructed by someone lax in their terminology. No
software I know calculates the true variogram (twice
the semi-variogram).

The correlogram is simply the semi-variogram upside
down and standardised to vary between -1 and +1. The
disadvantage of this approach (or the covariance
function is that it is difficult to assess the nugget
effect accurately.

provides essential information about teh variability

Isobel Clark
http://ecosse.ontheweb.com/whatsnew.htm

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