[R-sig-Geo] Problem with semi variance estimates in space time variogram (gstat)

[Edzer Pebesma]

I believe the sample variance of the data should equal the semivariance
averaged over all distances (in space and time).

You computed this over a very restricted (space time) range.

A large scale trend in the data could lead to a strong difference
between semivariances and sample variance; think of evaluating the
variogram of z=x+y, where x and y both range from 0 to 1000, and the
varioram is evaluated up to distance 10.

On 06/14/2012 05:47 PM, Rawlins, Barry G. wrote:
> Hello
>  
> The variograms function in gstat is returning semi-variance estimates
> which appear to be two orders of magnitude lower than what I expect from
> calculating the variance of the data in the attached space time object
> created with the STFDF function:
>  
> library(gstat)
> library(spacetime)
> breaks<-seq(0.5,12.5,1)
> var_all <- variogram(VMC~1,sp_time_var, bounds=breaks, cutoff = 12.5) #
> VMC is the numeric data to calculate a sp-time varigram
> plot(var_all) # note the low values ranges on the scale <0.15
> var(sp_time_var at data$VMC) # variance is 11.5
>  
> So I would expect the variance in the upper part of the graph to
> approach a value around 11.
>  
> Have I mis-specified the variogram function? I know from gstat that one
> had to specify a gstat object, but here I thought specifying the
> spacetime object was the right way to proceed and that VMC~1 was the
> correct syntax?
>  
> Any advice much appreciated.
> Barry
>  
>  
> 
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-- 
Edzer Pebesma
Institute for Geoinformatics (ifgi), University of Münster
Weseler Straße 253, 48151 Münster, Germany. Phone: +49 251
8333081, Fax: +49 251 8339763  http://ifgi.uni-muenster.de
http://www.52north.org/geostatistics      e.pebesma at wwu.de