NAME

       blockmedian - Block average (x, y, z) data tables by L1 norm

SYNOPSIS

       blockmedian [ table ] increment region [  ] [ [b] ] [ r|s[-] ] [  ] [ quantile ] [ [level]
       ] [ [i|o] ] [ -b<binary> ] [ -d<nodata> ] [ -f<flags> ] [ -h<headers> ] [  -i<flags>  ]  [
       -o<flags> ] [ -r ] [ -:[i|o] ]

       Note: No space is allowed between the option flag and the associated arguments.

DESCRIPTION

       blockmedian  reads  arbitrarily located (x,y,z) triples [or optionally weighted quadruples
       (x,y,z,w)] from standard input [or table] 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 FORMAT_FLOAT_OUT parameter in your gmt.conf file, or you may
       choose binary input and/or output to avoid loss of precision.

REQUIRED ARGUMENTS

       -Ixinc[unit][=|+][/yinc[unit][=|+]]
              x_inc [and optionally y_inc] is the  grid  spacing.  Optionally,  append  a  suffix
              modifier. Geographical (degrees) coordinates: Append m to indicate arc minutes or s
              to indicate arc seconds. If one of the units e,  f,  k,  M,  n  or  u  is  appended
              instead,  the  increment  is assumed to be given in meter, foot, km, Mile, nautical
              mile or US survey foot, respectively, and  will  be  converted  to  the  equivalent
              degrees  longitude  at the middle latitude of the region (the conversion depends on
              PROJ_ELLIPSOID). If /y_inc is given but set to 0 it will be reset equal  to  x_inc;
              otherwise  it  will  be  converted  to  degrees  latitude. All coordinates: If = is
              appended then the corresponding max x (east) or y (north) may be slightly  adjusted
              to  fit  exactly  the  given  increment  [by  default the increment may be adjusted
              slightly to fit the given domain]. Finally, instead of giving an increment you  may
              specify  the  number  of  nodes  desired  by  appending  +  to the supplied integer
              argument; the increment is then recalculated from  the  number  of  nodes  and  the
              domain.  The  resulting  increment  value  depends  on  whether you have selected a
              gridline-registered or pixel-registered grid;  see  App-file-formats  for  details.
              Note:  if -Rgrdfile is used then the grid spacing has already been initialized; use
              -I to override the values.

       -R[unit]xmin/xmax/ymin/ymax[r] (more ...)
              Specify the region of interest.

OPTIONAL ARGUMENTS

       table  3 [or 4, see -W] column ASCII data table] column ASCII file(s) [or binary, see -bi]
              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.

       -C     Use the center of the block as the output location [Default uses the median  x  and
              median y as location (but see -Q)].

       -E[b]  Provide  Extended  report  which  includes  s  (the L1 scale of the median), l, the
              lowest value,  and  h,  the  high  value  for  each  block.  Output  order  becomes
              x,y,z,s,l,h[,w].  [Default  outputs x,y,z[,w]. For box-and-whisker calculation, use
              -Eb which will output x,y,z,l,q25,q75,h[,*w*], where q25 and q75 are  the  25%  and
              75% quantiles, respectively. See -W for w output.

       -Er|s[-]
              Provide source id s or record number r output, i.e., append the source id or record
              number associated with the median value. If tied then report the record  number  of
              the  higher  of the two values; append - to instead report the record number of the
              lower value.  Note that both -E[b] and -Er[-] may be specified. For -Es  we  expect
              input  records  of  the form x,y,z[,w],sid, where sid is an unsigned integer source
              id.

       -Q     (Quicker) Finds median z and (x,y) at that the median z [Default  finds  median  x,
              median y independent of z]. Also see -C.

       -Tquantile
              Sets  the quantile of the distribution to be returned [Default is 0.5 which returns
              the median z]. Here, 0 < quantile < 1.

       -V[level] (more ...)
              Select verbosity level [c].

       -W[i|o]
              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].

       -bi[ncols][t] (more ...)
              Select native binary input. [Default is 3 (or 4 if -Wi is set)].

       -bo[ncols][type] (more ...)
              Select native binary output. [Default is 3 (or  4  if  -Wo  is  set)].  -E  adds  3
              additional columns.

       -d[i|o]nodata (more ...)
              Replace input columns that equal nodata with NaN and do the reverse on output.

       -f[i|o]colinfo (more ...)
              Specify data types of input and/or output columns.

       -h[i|o][n][+c][+d][+rremark][+rtitle] (more ...)
              Skip or produce header record(s).

       -icols[l][sscale][ooffset][,...] (more ...)
              Select input columns (0 is first column).

       -ocols[,...] (more ...)
              Select output columns (0 is first column).

       -r (more ...)
              Set  pixel  node registration [gridline]. Each block is the locus of points nearest
              the grid value location. For example, with  -R10/15/10/15  and  -I1:  with  the  -r
              option  10 <= (x,y) < 11 is one of 25 blocks; without it 9.5 <= (x,y) < 10.5 is one
              of 36 blocks.

       -:[i|o] (more ...)
              Swap 1st and 2nd column on input and/or output.

       -^ or just -
              Print a short message about the syntax of the command, then exits (NOTE: on Windows
              use just -).

       -+ or just +
              Print  an  extensive  usage  (help)  message,  including  the  explanation  of  any
              module-specific option (but not the GMT common options), then exits.

       -? or no arguments
              Print a complete usage (help) message, including the explanation of  options,  then
              exits.

       --version
              Print GMT version and exit.

       --show-datadir
              Print full path to GMT share directory and exit.

ASCII FORMAT PRECISION

       The  ASCII  output formats of numerical data are controlled by parameters in your gmt.conf
       file. Longitude and latitude are formatted  according  to  FORMAT_GEO_OUT,  whereas  other
       values are formatted according to FORMAT_FLOAT_OUT. Be aware that the format in effect can
       lead to loss of precision in the output, which can lead to various problems downstream. If
       you  find  the  output  is not written with enough precision, consider switching to binary
       output (-bo if available) or specify more decimals using the FORMAT_FLOAT_OUT setting.

EXAMPLES

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

              gmt blockmedian hawaii_b.xyg -R198/208/18/25 -I5m -bi3d > hawaii_5x5.xyg

       To  compute the shape of a data distribution per bin via a box-and-whisker diagram we need
       the 0%, 25%, 50%, 75%, and 100% quantiles. To do so on a global 5 by 5 degree  basis  from
       the ASCII table depths.xyz and send output to an ASCII table, run

              gmt blockmedian depths.xyz -Rg -I5 -Eb -r > depths_5x5.txt

SEE ALSO

       blockmean,  blockmode,  gmt, gmt.conf, greenspline, nearneighbor, surface, sphtriangulate,
       triangulate

COPYRIGHT

       2015, P. Wessel, W. H. F. Smith, R. Scharroo, J. Luis, and F. Wobbe