Hello. I have several fairly basic questions about geostatistical
analysis, interpretation and software. I received great advice from this
list on a methods question about a year ago (now I'm analyzing the data),
so I thought I'd try the list on these questions. I've floundered through
the archives, and some introductory books, but I'm still have some
questions. I don't think it should be relevant for these questions, but
just in case: I'm studying the ecology, distribution, dispersal of a
parasitic plant and the data are on host characteristics, parasite
prevalence etc. If you can help with even one of these questions (or
correct any erroneous assumptions I have), I would appreciate hearing. I
will summarize responses.
1) In R (S plus clone), I can calculated trend surfaces and used
the residuals to calculate detrended variograms or correlograms. In
Geoeas and Variowin I can calculate directional (setting angels and
tolerance) or omnidirectional variograms or correlograms.
How does a detrended variogram compare to an omnidirectional or
directional variogram? What are the pros and cons of these methods? (the
variograms from the two methods look quite different, although the
correlograms look quite similar)
2) What exactly is the difference/ pros and cons of correlograms
versus variograms? My correlograms look like almost pure nugget (except
perhaps at very small lags), but the detrended variograms have interesting
shapes - decreasing, upside down U, typical rise with plateau - how do I
interpret this? Does one give me more information than the other?
3) Where does kriging fit in? My understanding is that this is for
prediction (and uses the variogram). I am mainly interested in description
of patterns at this point.
4) How do cross variograms work? I have variables that are
correlated. Would cross variograms be appropriate?
5) How does one determine an appropriate degree polynomial to use
for trend surface analysis? Recommended reading for a beginner in trend
6) Software: Is there a way in R to determine how many pairs are
in each bin? Or to truncate variograms (it prints the entire distance and
it is my understanding that one shouldn't calculate variograms beyond half
the maximum distance).
Can SAS do spatial statistics? Is it part of the basic
package or is there some sort of add on? If you've tried it, do you
How do you make figures for publications? I like Geoeas and
Variowin because I can see what they are doing and I can change parameters
easily, but I have not been able to export figures. I find R frustrating
because I don't feel like I can't see what it is doing (I'm a serious
novice in R), but I can export the graphics. Any suggestions? (I don't have
funds to buy software and I'm running out of time/patience for learning a
lot of new software for this particular project).
Thank you very much.
Ecology and Evolutionary Biology
University of Arizona
Tucson, AZ 85721
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