Problem with semi variance estimates in space time variogram (gstat)

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
 
 

-- 
This message (and any attachments) is for the recipient only. NERC
is subject to the Freedom of Information Act 2000 and the contents
of this email and any reply you make may be disclosed by NERC unless
it is exempt from release under the Act. Any material supplied to
NERC may be stored in an electronic records management system.

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



--
This message (and any attachments) is for the recipient only. NERC
is subject to the Freedom of Information Act 2000 and the contents
of this email and any reply you make may be disclosed by NERC unless
it is exempt from release under the Act. Any material supplied to
NERC may be stored in an electronic records management system.

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
 
 

-- 
This message (and any attachments) is for the recipient only. NERC
is subject to the Freedom of Information Act 2000 and the contents
of this email and any reply you make may be disclosed by NERC unless
it is exempt from release under the Act. Any material supplied to
NERC may be stored in an electronic records management system.

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
 
 

-- 
This message (and any attachments) is for the recipient only. NERC
is subject to the Freedom of Information Act 2000 and the contents
of this email and any reply you make may be disclosed by NERC unless
it is exempt from release under the Act. Any material supplied to
NERC may be stored in an electronic records management system.