BLOCKMEDIAN(l)															    BLOCKMEDIAN(l)

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] ] [ -L ] [ -Q ] [ -V ] [ -W[io]
       ] [ -: ] [ -bi[s][n] ] [ -bo[s][n] ]

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] 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     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

       -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 registration.] (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 .gmtdefaults file. If used, GMT default
	      is 1 header record.  Not used with binary data.

       -L     Indicates that the x column contains longitudes, which may differ from the region in -R  by  [multiples  of]  360  degrees  [Default
	      assumes no periodicity].

       -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)].  Applies to geo-
	      graphic coordinates only.

       -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 precision 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(1gmt), blockmode(1gmt), gmt(1gmt), gmtdefaults(1gmt), nearneighbor(1gmt), surface(1gmt), triangulate(1gmt)

								    1 Jan 2004							    BLOCKMEDIAN(l)