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


       blockmode      [      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 ]


       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 .gmtdefaults4 file, or
       you may choose binary input and/or output using single or double preci-
       sion storage.

              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

       -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 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  .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  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  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 mode estimates from the double precision
       binary data in hawaii_b.xyg and output an ASCII table, run:

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


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

GMT4.0                            1 Oct 2004                      BLOCKMODE(l)

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