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BLOCKMEAN(1)		     Generic Mapping Tools		  BLOCKMEAN(1)

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

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

DESCRIPTION
       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 rou‐
       tines 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, blockmean 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 mean location].

       -E     Provide Extended report which includes s (the standard deviation
	      of  the  mean),  l,  the lowest value, and h, the high value for
	      each block. Output order becomes x,y,z,s,l,h[,w]. [Default  out‐
	      puts x,y,z[,w].  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.

       -S     Use -Sz to report the sum of all z-values inside a block, or -Sw
	      to report the sum of weights [Default reports  mean  value].  If
	      -Sw is selected and no weights are supplied (i.e., no -W given),
	      then the weight sum will equal the number of points inside  each
	      block.

       -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  mean  values  from  the  ASCII  data  in
       hawaii.xyg, run

       blockmean hawaii.xyg -R 198/208/18/25 -I 5m > hawaii_5x5.xyg

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

GMT 4.5.14			  1 Nov 2015			  BLOCKMEAN(1)
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