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] ] [ -Q ] [ -V  ]  [
       -W[io] ] [ -: ] [ -bi[s][n] ] [ -bo[s][n] ] [ -f[i|o]colinfo ]


       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 .gmtdefaults4 file, or you may
       choose binary input and/or output  using  single  or  double  precision

              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

       -I     x_inc [and optionally y_inc] is the grid spacing.  Append  m  to
              indicate minutes or c to indicate seconds.

       -R     xmin,  xmax,  ymin, and ymax specify the Region of interest. For
              geographic regions,  these  limits  correspond  to  west,  east,
              south,  and north and you may specify them in decimal degrees or
              in [+-]dd:mm[][W|E|S|N] format. Append r  if  lower  left
              and  upper  right map coordinates are given instead of wesn. The
              two shorthands  -Rg  -Rd  stand  for  global  domain  (0/360  or
              -180/+180  in longitude respectively, with -90/+90 in latitude).
              For calendar time coordinates you may either give relative  time
              (relative  to  the  selected  TIME_EPOCH  and  in  the  selected
              TIME_UNIT; append t to -JX|x), or  absolute  time  of  the  form
              [date]T[clock]  (append  T  to  -JX|x). At least one of date and
              clock must be present; the T is always required. The date string
              must  be  of  the form [-]yyyy[-mm[-dd]] (Gregorian calendar) or
              yyyy[-Www[-d]] (ISO week calendar), while the clock string  must
              be  of  the form hh:mm:ss[.xxx]. The use of delimiters and their
              type and positions must be as indicated  (however,  input/output
              and plotting formats are flexible).


       -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  .gmtdefaults4  file.  If  used,  GMT
              default  is  1  header record. Use -Hi if only input data should
              have header records [Default will write out  header  records  if
              the input data have them].  Not used with binary data.

       -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  and/or output. [Default is (longitude,latitude)].  Append
              i to select input only or o  to  select  output  only.  [Default
              affects both].

       -bi    Selects  binary input. Append s for single precision [Default is
              double].  Append n for the  number  of  columns  in  the  binary
              [Default is 3 (or 4 if -W is set) columns].

       -bo    Selects binary output. Append s for single precision [Default is
              double].  Append n for the  number  of  columns  in  the  binary

       -f     Special  formatting  of  input  and output columns (time or geo-
              graphical data) Specify i(nput) or  o(utput)  [Default  is  both
              input  and output].  Give one or more columns (or column ranges)
              separated by commas.  Append T (Absolute calendar time), t (time
              relative  to  chosen TIME_EPOCH), x (longitude), y (latitude), g
              (geographic coordinate), or f (floating point) to each column or
              column range item.


       To  find  5  by 5 minute block medians from the double precision binary
       data in hawaii_b.xyg and output an ASCII table, run

       blockmedian hawaii_b.xyg -R198/208/18/25 -I5m -bi3 > hawaii_5x5.xyg


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

GMT4.0                            1 Oct 2004                    BLOCKMEDIAN(l)

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