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 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  .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  dif-
              fer 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
              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) columns].

       -O     Selects  Overlay  plot mode [Default initializes a new plot sys-
              tem].


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(l),  blockmode(l),  gmt(l),  gmtdefaults(l), nearneighbor(l),
       surface(l), triangulate(l)



GMT3.4.6                          1 Jan 2005                    BLOCKMEDIAN(l)

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