NAME
s.medp - Median polish for a GRASS sites list.
(GRASS Sites Program)
SYNOPSIS
s.medp
s.medp help
s.medp [-aeqs] sites=name
vect=name
output=name
[report=name] [thresh=n]
DESCRIPTION
s.medp performs median polish on an existing sites
list. The sites are overlayed onto a grid
(vect=name) and each site is associated
with the closest node. Therefore, sites do not necessarily
have to be oriented on a grid. Each node may be associated
with zero, one, or more sites and the grid may be rotated.
Also, the length of each box does not have to be equal to
width of each box defining the grid.
Once sites are associated with nodes on a p by
q grid, p+q+1 extra storage locations are
created (initialized to zero) to store all,
row, and column effects. The median of
each row is removed from the data and added to the extra
p cells. Then, medians are removed from the data
as well as from the p cells containing
row effects. The medians of data in this pass
are stored in the extra q cells (column
effects) and the median of the row effects is stored in the
extra (p+1, q+1) cell (all effect). This
is repeated until each successive iteration leaves each
site unchanged (within a tolerance, described by
[thresh=n]).
OPTIONS
Flags:
- -a
- Use all sites found in the named sites file, rather than
limiting output to sites falling within the current geographic region.
- -e
- Store row, column, and all effects in output file.
- -s
- Write results to a sites list file (default is to write
points in a binary vector file).
- -q
- Quiet. Cut out the chatter.
Parameters:
- sites=name
- Name of an existing sites file.
- vect=name
- Name of the grid file (binary vector file).
- output=name
- Name of the output file (binary vector file or sites list).
- report=name
- Name of an ASCII file which shows original and residual data in
tabular form.
- thresh=n
- Threshold to determine when convergence of median polish is obtained.
(default = 1).
s.medp can be run either non-interactively or
interactively. The program will be run non-interactively
if the user specifies the name of an existing
sites list file, name for a vect file,
and name of an output file using the form
-
s.medp [-aeqs] sites=name
vect=name
output=name
[report=name] [thresh=n]
Alternately, the user can simply type s.medp on the command line,
without program arguments. In this case, the user will be prompted for
parameter values using the standard GRASS
parser interface.
NOTES
When using the report option, the tabular report may
be more than 128 columns wide. Therefore, it may be useful
to add a TeX or PostScript wrapper before
printing (e.g., enscript -h -r -fCourier-Bold5 -p
report.ps report) so that the page is in landscape
orientation and a very small font is used.
The input vector file defining the grid should be large
enough so that the extreme east-west and north-south nodes
(for non-rotated grids) can be used to store row and column
effects. Otherwise, when residuals and effects are
written to the output file, residuals may share the same
spatial location as effects when written to a vector output
file.
Currently, nothing is done with the residuals, but I would
like implement some sort of analysis of these in the future
(suggestions?). Kriging using these residuals as new data
set is being planned.
SEE ALSO
v.mkgrid
v.transform
parser
Cressie, Noel A. C. (1991).
Statistics for Spatial Data,
New York, NY: John Wiley & Sons.
pp. 186-187.
BUGS
This should still be consider beta-release software.
Although I have repeated results shown in Cressie (1991), I
would greatly appreciate someone testing this further.
The -s flag is yet to be implemented.
This should probably be re-written to use points in a
vector file as input (thus creating v.medp).
Ideally, I would also like to output three data files for
plotting by a graphing program (e.g., gnuplot),
but I never got around to doing this. The classic plot is
to show three surfaces: data = trend + residuals. The
trend can be obtained by extrapolation and/or
interpolation.
Extrapolating and interpolating using the row, column, and
all effects to create a raster file would be a nice feature
to include in the future. Instead, I am considering
retrieving the effects from the output file and creating
the surface with separate program.
Please send all bug fixes and comments to the author.
AUTHOR
James Darrell McCauley, Agricultural
Engineering Purdue University
Last changed: $Date$