NAME

       blockmode - filter to block average (x,y,z) data by mode estimation.


SYNOPSIS

       blockmode      [      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

       blockmode  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 mode estimates of position and value for
       every non-empty block in a grid region defined by the -R and  -I  argu-
       ments.  Either blockmean, blockmedian, or blockmode should be used as a
       pre-processor before running surface  to  avoid  aliasing  short  wave-
       lengths.  These  routines  are  also generally useful for decimating or
       averaging (x,y,z) data. You can modify the precision of the output for-
       mat 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,  blockmode  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 modal xy 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 mode z and mean (x, y) [Default  finds  mode  x,
              mode y, mode 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 modal 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 mode estimates from  the  double  precision
       binary data in hawaii_b.xyg and output an ASCII table, try

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


SEE ALSO

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



GMT3.4.6                          1 Jan 2005                      BLOCKMODE(l)

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