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       blockmean - filter to block average (x,y,z) data by L2 norm


       blockmean  [ xyz[w]file(s) ] -Ix_inc[m|c][/y_inc[m|c]] -Rwest/east/south/north[r] [ -C ] [
       -F ] [ -H[nrec] ] [ -L ] [ -S ] [ -V ] [ -W[io] ] [ -: ] [ -bi[s][n] ] [ -bo[s][n] ]


       blockmean 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 mean
       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.

              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,  blockmean  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.


       -C     Use the center of  the  block  as  the  output  location  [Default  uses  the  mean

       -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].

       -S     Report the sum of all z-values inside a block [Default reports mean value].

       -V     Selects verbose mode, which will send progress  reports  to  stderr  [Default  runs

       -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  mean
              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

       -bo    Selects binary output. Append s for single precision [Default is double].


       To find 5 by 5 minute block means from the ASCII data in hawaii.xyg, try

       blockmean hawaii.xyg -R198/208/18/25 -I5m > hawaii_5x5.xyg


       blockmedian(1gmt),  blockmode(1gmt),  gmtdefaults(1gmt),  gmt(1gmt),   nearneighbor(1gmt),
       surface(1gmt), triangulate(1gmt)

                                            1 Jan 2004                               BLOCKMEAN(l)