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] 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

      -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 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 -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 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, blockmode, gmt, gmtdefaults, nearneighbor, surface,
      triangulate