NAME
blockmedian - filter to block average (x,y,z) data by L1
norm.
SYNOPSIS
blockmedian [ xyz[w]file(s) ] - Ix_inc[m|c][/y_inc[m|c]] -
Rwest/east/south/north[r] [ -H ] [ -N ] [ -V ] [ -W[io] ] [
-: ] [ -b[i|o][d] ]
DESCRIPTION
blockmedian reads arbitrarily located (x,y,z) triples [or
optionally weighted quadruples (x,y,z,w)] from standard
input [or xyz[w]file(s)] and writes to standard output a
median position and value for every non-empty block in a
grid region defined by the -R and - I arguments. Either
blockmean or blockmedian should be used as a pre-processor
before running surface to avoid aliasing short wavelengths.
These routines are also generally useful for decimating or
averaging (x,y,z) data. You can modify the precision of the
output format by editing the D_FORMAT parameter in your
.gmtdefaults file, or you may choose binary input and/or
output using single or double precision storage.
xyz[w]file(s)
3 [or 4] column ASCII file(s) [or binary, see -b] hold-
ing (x,y,z[,w]) data values. [w] is an optional weight
for the data. If no file is specified, blockmedian
will read from standard input.
-I x_inc [and optionally y_inc] is the grid spacing.
Append m to indicate minutes or c to indicate seconds.
-R west, east, south, and north specify the Region of
interest. To specify boundaries in degrees and minutes
[and seconds], use the dd:mm[:ss] format. Append r if
lower left and upper right map coordinates are given
instead of wesn.
OPTIONS
-H Input file(s) has Header record(s). Number of header
records can be changed by editing your .gmtdefaults
file. If used, GMT default is 1 header record. Not
used with binary data.
-N Offsets the grid registration of the boxes. Use pixel
registration instead of grid registration. By default,
the centers of the blocks coincide with the defined
lattice nodes; each block contains all points nearest
to a lattice node. The - N option will offset the
blocks so their edges coincide with the lattice lines.
For example, suppose the arguments are - R10/15/10/15
and - I1 This defines a 6 x 6 block array; the lower
left block will average data within 9.5 <= (x,y) <
10.5. If the same lattice is given with the -N option,
the block array is 5 x 5 and the lower left block will
average the data within 10 <= (x,y) < 11. The default
registration is normally used for preprocessing and
gridding map data.
-V Selects verbose mode, which will send progress reports
to stderr [Default runs "silently"].
-W Weighted modifier[s]. Unweighted input and output has
3 columns x,y,z; Weighted i/o has 4 columns x,y,z,w.
Weights can be used in input to construct weighted
median values in blocks. Weight sums can be reported
in output for later combining several runs, etc. Use -
W for weighted i/o, -Wi for weighted input only, -Wo
for weighted output only. [Default uses unweighted
i/o]
- : Toggles between (longitude,latitude) and
(latitude,longitude) input/output. [Default is
(longitude,latitude)]
-b Selects binary input and output mode. Append i or o
to select single precision binary input or output only.
[Default is ASCII]. Append d to indicate double preci-
sion for binary data (if both binary input and output
are selected they will have the same precision).
EXAMPLES
To find 5 by 5 minute block medians from the double preci-
sion binary data in hawaii_b.xyg and output an ASCII table,
try
blockmedian hawaii_b.xyg - R198/208/18/25 - I5m - bid >
hawaii_5x5.xyg
SEE ALSO
blockmean, gmt, gmtdefaults, nearneighbor, surface, triangu-
late
REFERENCES
Wessel, P., and W. H. F. Smith, 1995, The Generic Mapping
Tools (GMT) version 3.0 Technical Reference & Cookbook,
SOEST/NOAA.
Wessel, P., and W. H. F. Smith, 1995, New Version of the
Generic Mapping Tools Released, EOS Trans. AGU, 76, p. 329.
Wessel, P., and W. H. F. Smith, 1995, New Version of the
Generic Mapping Tools Released,
http://www.agu.org/eos_elec/95154e.html, Copyright 1995 by
the American Geophysical Union.
Wessel, P., and W. H. F. Smith, 1991, Free Software Helps
Map and Display Data, EOS Trans. AGU, 72, p. 441.