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] hold-
          ing (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 Cook-
          book 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  preci-
     sion  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)