On 13.02.2014 14:00, Edzer Pebesma wrote: > On 02/13/2014 01:30 PM, Roelof Coster wrote: >> When I make the sample space-time variogram (with variogramST), it >> automatically chooses a time-lag difference of about 2 days. This is much >> too small to be meaningful for my data; half-year periods would be >> interesting. Is there a way to tell this to the variogramST function? > > variogramST (in package gstat) assumes that the time gap between > observations is constant. If you have "approximately every half year" > observations, are some of them only a few days apart? Did you see a > warning printed? > > If the time differences are very un-even, the whole activity may be > pretty meaningless (with current implementation), and a mapping to > stricter regularity might help. We have coding Friday tomorrow, drop by > or send something useful and we might be able to work it out. A "quick" work around along Edzer's suggestion could be the following: Currently, variogramST calculates the empirical variogram per temporal lag tlag for pairs of spatial slices such as (STFDF[,i], STFDF[,i+tlag]) while tlag is from a vector like tlags=0:5. Given your data has been collected within a couple of days every half year and encoded as such, you might want to use fake dates that set all these entries to the same point in time, i.e. the same temporal index. This way, STFDF[,i] and STFDF[,i+1] would return two time slices half a year (in fake dates) apart from each other and that would be the temporal width variorgamST would be using as well. Otherwise, lets say your data has continuously be recorded while a sensor moved across the Netherlands revisiting each point every half year, some more careful thinking would be needed.
However, the usefulness of spatio-temporal kriging depends on the spatio-temporal properties of the process you are looking at (i.e. will there be any explanatory value between observations separated half a year). HTH, Ben > _______________________________________________ > R-sig-Geo mailing list > R-sig-Geo@r-project.org > https://stat.ethz.ch/mailman/listinfo/r-sig-geo > -- Benedikt Gräler ifgi - Institute for Geoinformatics University of Muenster http://ifgi.uni-muenster.de/graeler Phone: +49 251 83-33082 Mail: ben.grae...@uni-muenster.de
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