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 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].

       -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)