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BLOCKMEDIAN(1)		     Generic Mapping Tools		BLOCKMEDIAN(1)

NAME
       blockmedian - filter to block average (x,y,z) data by L1 norm.

SYNOPSIS
       blockmedian   [	 xyz[w]file(s)	]  -Ixinc[unit][=|+][/yinc[unit][=|+]]
       -Rxmin/xmax/ymin/ymax[r] [ -C ] [ -E[b] ] [ -F ] [ -H[i][nrec] ] [ -Q ]
       [    -Tquantile	 ]   [	 -V   ]	  [   -W[io]   ]   [   -:[i|o]	 ]   [
       -b[i|o][s|S|d|D[ncol]|c[var1/...]] ] [ -f[i|o]colinfo ]

DESCRIPTION
       blockmedian 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 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 D_FORMAT parameter in your .gmtdefaults4 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, blockmedian will read from stan‐
	      dard input.

       -I     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 c to indicate arc seconds.
	      If one of the units e, k, i,  or	n  is  appended	 instead,  the
	      increment	 is assumed to be given in meter, km, miles, or nauti‐
	      cal miles, respectively, and will be converted to the equivalent
	      degrees longitude at the middle latitude of the region (the con‐
	      version depends on 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  Appendix  B	for  details.  Note: if -Rgrdfile is used then
	      grid spacing has already been initialized; use  -I  to  override
	      the values.

       -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[:ss.xxx][W|E|S|N] format.  Append r if  lower  left
	      and  upper  right	 map coordinates are given instead of w/e/s/n.
	      The two shorthands -Rg and -Rd stand for	global	domain	(0/360
	      and  -180/+180  in longitude respectively, with -90/+90 in lati‐
	      tude).  Alternatively, specify the name of an existing grid file
	      and the -R settings (and grid spacing, if applicable) are copied
	      from the grid.  For calendar time	 coordinates  you  may	either
	      give  (a) relative time (relative to the selected TIME_EPOCH and
	      in the selected TIME_UNIT; append t to -JX|x), or	 (b)  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 delim‐
	      iters and their type and positions must be exactly as  indicated
	      (however,	 input,	 output and plot formats are customizable; see
	      gmtdefaults).

OPTIONS
       -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     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.

       -F     Force pixel node registration  [Default  is  gridline  registra‐
	      tion].  (Node 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  -R
	      10/15/10/15 and and -I 1:	 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).  If used, the default number
	      of  header records is N_HEADER_RECS.  Use -Hi if only input data
	      should have  header  records  [Default  will  write  out	header
	      records  if  the	input  data  have them]. Blank lines and lines
	      starting with # are always skipped.  Not used with binary data.

       -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.

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

       -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 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  inputonly,  -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
	      d	 (double)].   Uppercase	 S  or	D  will	 force	byte-swapping.
	      Optionally, append ncol, the number of columns  in  your	binary
	      input  file if it exceeds the columns needed by the program.  Or
	      append c	if  the	 input	file  is  netCDF.  Optionally,	append
	      var1/var2/...  to specify the variables to be read.  [Default is
	      3 (or 4 if -Wi is set)].

       -bo    Selects binary output.  Append s for single  precision  [Default
	      is  d  (double)].	  Uppercase  S	or D will force byte-swapping.
	      Optionally, append ncol, the number of desired columns  in  your
	      binary  output  file.   [Default	is 3 (or 4 if -Wo is set)]. -E
	      adds 3 additional columns.

       -f     Special formatting of input and/or output columns (time or  geo‐
	      graphical	 data).	  Specify  i  or  o to make this apply only to
	      input or output [Default applies to both].   Give	 one  or  more
	      columns (or column ranges) separated by commas.  Append T (abso‐
	      lute calendar time), t (relative time in chosen TIME_UNIT	 since
	      TIME_EPOCH),  x (longitude), y (latitude), or f (floating point)
	      to each column or column range item.  Shorthand  -f[i|o]g	 means
	      -f[i|o]0x,1y (geographic coordinates).

ASCII FORMAT PRECISION
       The ASCII output formats of numerical data are controlled by parameters
       in your .gmtdefaults4  file.   Longitude	 and  latitude	are  formatted
       according  to  OUTPUT_DEGREE_FORMAT, whereas other values are formatted
       according to D_FORMAT.  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	preci‐
       sion, consider switching to binary output (-bo if available) or specify
       more decimals using the D_FORMAT 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

       blockmedian hawaii_b.xyg -R 198/208/18/25 -I 5m -bi 3 > 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

       blockmedian depths.xyz -Rg -I 5 -F -Eb > depths_5x5.txt

SEE ALSO
       blockmean(1), blockmode(1),  GMT(1),  gmtdefaults(1),  nearneighbor(1),
       surface(1), triangulate(1)

GMT 4.5.14			  1 Nov 2015			BLOCKMEDIAN(1)
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