[R] problems with variogram
Jon Olav Skoien
j.skoien at geo.uu.nl
Wed Jun 4 15:03:32 CEST 2008
Laura Saltyte wrote:
> Hello,
>
> I have data at 10 locations, in each location there are time series
> (T=56). Question is: when I’m 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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