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       blockmedian - filter to block average (x,y,z) data by L1 norm.


       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] ]


       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.

              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.


       -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

       -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 geographic 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)

       -bo    Selects binary output. Append s for single precision [Default is double].


       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


       blockmean(1gmt),  blockmode(1gmt),   gmt(1gmt),   gmtdefaults(1gmt),   nearneighbor(1gmt),
       surface(1gmt), triangulate(1gmt)

                                            1 Jan 2004                             BLOCKMEDIAN(l)