[R-sig-Geo] Spacetime regression

[Benedikt Gräler]

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

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-- 
Benedikt Gräler

ifgi - Institute for Geoinformatics
University of Muenster

http://ifgi.uni-muenster.de/graeler

Phone: +49 251 83-33082
Mail: ben.graeler at uni-muenster.de

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