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blockmedian(1)                        GMT                       blockmedian(1)




NAME

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


SYNOPSIS

       blockmedian [ table ]
        -Iincrement
        -Rregion  [   -C  ] [  -E[b] ] [  -Er|s[-] ] [  -Q ] [  -Tquantile ] [
       -V[level] ] [  -W[i|o][+s] ] [ -bbinary ] [ -dnodata ] [ -eregexp  ]  [
       -fflags ] [ -hheaders ] [ -iflags ] [ -oflags ] [ -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 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 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][+e|n][/yinc[unit][+e|n]]
              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, nau-
              tical mile or US survey foot, respectively,  and  will  be  con-
              verted  to  the equivalent degrees longitude at the middle lati-
              tude 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  coordi-
              nates:  If +e 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 +n to
              the supplied integer argument; the increment  is  then  recalcu-
              lated  from  the  number  of nodes and the domain. The resulting
              increment value depends on whether you  have  selected  a  grid-
              line-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.

       -Rxmin/xmax/ymin/ymax[+r][+uunit] (more a|)
              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, blockme-
              dian 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 val-
              ues;  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 a|)
              Select verbosity level [c].

       -W[i|o][+s]
              Weighted modifier[s]. Unweighted input and output have 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  for  each  block.
              Weight  sums  can be reported in output for later combining sev-
              eral runs, etc. Use -W for weighted i/o, -Wi for weighted  input
              only, and -Wo for weighted output only. [Default uses unweighted
              i/o]. If your weights are  actually  uncertainties  (one  sigma)
              then append +s and we compute weight = 1/sigma.

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

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

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

       -e[~]^<i>apattern^<i>a | -e[~]/regexp/[i] (more a|)
              Only accept data records that match the given pattern.

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

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

       -icols[+l][+sscale][+ooffset][,^<i>a|] (more a|)
              Select input columns and transformations (0 is first column).

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

       -r (more a|)
              Set pixel node registration [gridline]. Each block is the  locus
              of  points  nearest the grid value location. Consider an 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 a|)
              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 just use -).

       -+ or just +
              Print  an extensive usage (help) message, including the explana-
              tion 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 all options, then exits.


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,  absolute  time  is  under  the  control  of   FOR-
       MAT_DATE_OUT  and FORMAT_CLOCK_OUT, whereas general floating point val-
       ues are formatted according to FORMAT_FLOAT_OUT. Be aware that the for-
       mat  in effect can lead to loss of precision in ASCII 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  set-
       ting.


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% quan-
       tiles. 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(1),  blockmode(1),  gmt(1),  gmt.conf(5), greenspline(1),
       nearneighbor(1), surface(1), sphtriangulate(1), triangulate(1)


COPYRIGHT

       2017, P. Wessel, W. H. F. Smith, R. Scharroo, J. Luis, and F. Wobbe



5.4.2                            Jun 24, 2017                   blockmedian(1)

gmt5 5.4.2 - Generated Wed Jun 28 15:12:59 CDT 2017
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