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[nrec] ] [ -N ] [ -Q ] [ -V ]
[ -W[io] ] [ -: ] [ -bi[s][n] ] [ -bo[s] ]
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, blockmedian, or blockmode 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 Block centers have pixel registration. [Default: grid
registration.] (Registrations are defined in GMT Cook-
book Appendix B on grid file formats.) Each block is
the locus of points nearest the grid value location.
For example, with -R10/15/10/15 and and -I1: with the
-N option 10 <= (x,y) < 11 is one of 25 blocks; without
it 9.5 <= (x,y) < 10.5 is one of 36 blocks.
-Q (Quicker) Finds median z and (x, y) at that z [Default
finds median x, median y, median z].
-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)].
-bi Selects binary input. Append s for single precision
[Default is double]. Append n for the number of
columns in the binary file(s). [Default is 3 (or 4 if
-W is set) columns].
-bo Selects binary output. Append s for single precision
[Default is double].
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 -bi3 >
hawaii_5x5.xyg
SEE ALSO
blockmean(l), blockmode(l), gmt(l), gmtdefaults(l),
nearneighbor(l), surface(l), triangulate(l)