NAME
s.sv - Sample semivariogram of a GRASS sites list.
(GRASS Sites Program)
SYNOPSIS
s.sv
s.sv help
s.sv [-alq]
[sites=name]
lag=value
[lagtol=value]
[direction=value]
[angtol=value]
[graph=name]
[field=value]
DESCRIPTION
s.sv calculates a sample semivariogram and either plots
it using gnuplot or writes it to standard output.
For more information, refer to the tutorial or see
the example below.
OPTIONS
Flags:
- -q
- Quiet. Cut out the chatter.
- -p
- Plot the sample semivariogram (requires gnuplot).
- field
- Number of z-field attribute to use for calculation
- default: 1
Parameters:
- sites=name
- Name of an existing sites file. Default is standard input
with no field separators.
- lag=value
- Nominal lag distance.
- lagtol=value
- Tolerance on lag distance. Default is half of nominal distance.
- direction=value
- Direction of semivariogram. Default is omnidirectional semivariogram.
- angtol=value
- Angular tolerance on direction.
- graph=name
- Basename to save graphing data/commands files.
Graphs are saved in the current working directory with
the extensions .gp and .dat. Implies
the -p flag. If unspecified, semivariogram is
written to standard output.
NOTES
The plotting program can be selected by setting the environment
variable GRASS_GNUPLOT .
Without the -p flag, three columns of
data are written to standard output: lag distance (h),
semivariogram value (gamma), and the number of data pairs
used to compute it (N(h)). When the graph parameter
is set, these same three columns of data are
written to name.dat. Therefore, to replot
a sample semivariogram, use:
- gnuplot name.gp
To plot a histogram of N(h), simply edit name.gp
and redo the previously given command.
For more information, refer to the
tutorial.
SEE ALSO
s.univar
s.surf.krig
s.normal
m.svfit
gnuplot
Semivariogram Modeling - A GRASS Tutorial on
Exploratory Data Analysis and Semivariogram Modeling.
BUGS
Will not work correctly with lat-long data.
Should G_azimuth() be used to
calculate the angle between points?
Only Matheron's classical estimator is available with s.sv.
Others may be added in the future.
AUTHOR
James Darrell McCauley, Agricultural
Engineering, Purdue University
Last changed: $Date$