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

[Edzer Pebesma]

according to the documentation of ?variogram, there is a tlags argument.

On 06/15/2012 05:20 PM, Rawlins, Barry G. wrote:
> Hello Edzer
> 
> How do I alter the time lags used in the variogram call when I am calculating a space time variogram? I could not see any description of the way to do this in the variograms help page.
> 
> Thanks, Barry
> 
> -----Original Message-----
> From: r-sig-geo-bounces at r-project.org [mailto:r-sig-geo-bounces at r-project.org] On Behalf Of Edzer Pebesma
> Sent: 14 June 2012 23:06
> To: r-sig-geo at r-project.org
> Subject: Re: [R-sig-Geo] Problem with semi variance estimates in space time variogram (gstat)
> 
> 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