Re: [R] problems with variogram

From: Jon Olav Skoien <j.skoien_at_geo.uu.nl>
Date: Wed, 04 Jun 2008 15:03:32 +0200

Laura Saltyte wrote:
> Hello,
>
> I have data at 10 locations, in each location there are time series
> (T=56). Question is: when Iím fitting variogram what happens with those
> measures in each location? Are they taken as repeated measures? It's very
> important for my to know this
>
> Thanks a lot
>
Hi Laura,

I think this depends on which package you use for your variogram function. In variogram in package gstat, you can define your wanted behavior by manipulating the dX-parameter. Generally, the easiest way you can find an answer to your question if you dont get any help from the help-file of your variogram function, is to look at the number of pairs in the calculated sample variogram; - if you have relatively few pairs altogether (45 or less for your 10 locations, depending on the cutoff for the variogram function), the variogram was calculated from the average of the observations, or from one of the values (probably the first)
- if the number of pairs is lower than 45*56 (the length of your time series), the variogram was probably calculated as an average of the variograms from each separate time step
- if the number of pairs is "high", the variogram was probably calculated by comparing all observations independent of time steps with each other.

If you realize that your function uses the last method, I would recommend you to switch to one of the first two. Unless the temporal correlation is very high, the spatial correlation is likely to be (strongly) underestimated in this case.

For questions about spatial statistics, the list r-sig-geo will often be more useful than r-help.

Good luck,
Jon



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