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] [ -C ] [ -F ] [ -H[nrec] ] [ -Q ] [ -V ] [
-W[io] ] [ -: ] [ -bi[s][n] ] [ -bo[s][n] ] [ -f[i|o]colinfo ]
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 .gmtdefaults4 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] holding
(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 xmin, xmax, ymin, and ymax specify the Region of interest. For
geographic regions, these limits correspond to west, east,
south, and north and you may specify them in decimal degrees or
in [+-]dd:mm[:ss.xxx][W|E|S|N] format. Append r if lower left
and upper right map coordinates are given instead of wesn. The
two shorthands -Rg -Rd stand for global domain (0/360 or
-180/+180 in longitude respectively, with -90/+90 in latitude).
For calendar time coordinates you may either give relative time
(relative to the selected TIME_EPOCH and in the selected
TIME_UNIT; append t to -JX|x), or absolute time of the form
[date]T[clock] (append T to -JX|x). At least one of date and
clock must be present; the T is always required. The date string
must be of the form [-]yyyy[-mm[-dd]] (Gregorian calendar) or
yyyy[-Www[-d]] (ISO week calendar), while the clock string must
be of the form hh:mm:ss[.xxx]. The use of delimiters and their
type and positions must be as indicated (however, input/output
and plotting formats are flexible).
OPTIONS
-C Use the center of the block as the output location [Default uses
the median location (but see -Q)]. -C overrides -Q.
-F Block centers have pixel registration. [Default: grid registra-
tion.] (Registrations are defined in GMT Cookbook 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 -F option 10 <= (x,y) < 11 is one of 25 blocks;
without it 9.5 <= (x,y) < 10.5 is one of 36 blocks.
-H Input file(s) has Header record(s). Number of header records can
be changed by editing your .gmtdefaults4 file. If used, GMT
default is 1 header record. Use -Hi if only input data should
have header records [Default will write out header records if
the input data have them]. Not used with binary data.
-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 and/or output. [Default is (longitude,latitude)]. Append
i to select input only or o to select output only. [Default
affects both].
-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]. Append n for the number of columns in the binary
file(s).
-f Special formatting of input and output columns (time or geo-
graphical data) Specify i(nput) or o(utput) [Default is both
input and output]. Give one or more columns (or column ranges)
separated by commas. Append T (Absolute calendar time), t (time
relative to chosen TIME_EPOCH), x (longitude), y (latitude), g
(geographic coordinate), or f (floating point) to each column or
column range item.
EXAMPLES
To find 5 by 5 minute block medians from the double precision binary
data in hawaii_b.xyg and output an ASCII table, run
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)
GMT4.0 1 Oct 2004 BLOCKMEDIAN(l)
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