NAME

       blockmean - Block average (x, y, z) data tables by L2 norm

SYNOPSIS

       blockmean [ table ]
        -Iincrement
        -Rregion [  -C ] [  -E[p] ] [  -S[m|n|s|w] ] [  -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

       blockmean  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  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 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, 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 +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 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.

       -Rxmin/xmax/ymin/ymax[+r][+uunit] (more …)
              Specify the region of interest.

OPTIONAL ARGUMENTS

       table  3 [or 4, see -W] column ASCII data table  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, blockmean will read from standard input.

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

       -E[p]  Provide  Extended  report which includes s (the standard deviation about 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  outputs x,y,z[,w]. See -W for w output.  If -Ep is used
              we assume weights are 1/(sigma squared) and s becomes the propagated error  of  the
              mean.

       -S[m|n|s|w]
              Use  -Sn to report the number of points inside each block, -Ss to report the sum of
              all z-values inside a block, -Sw to report the sum  of  weights  [Default  (or  -Sm
              reports mean value].

       -V[level] (more …)
              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 mean
              values  for  each  block. Weight sums can be reported in output for later combining
              several 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 …)
              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.  The -Sn option will work with only 2 input columns (x and y).

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

       -e[~]”pattern” | -e[~]/regexp/[i] (more …)
              Only accept data records that match the given pattern.

       -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 and transformations (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. 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 …)
              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  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  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 FORMAT_DATE_OUT and FORMAT_CLOCK_OUT, whereas general floating point
       values are formatted according to FORMAT_FLOAT_OUT. Be aware that the format 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 setting.

EXAMPLES

       To find 5 by 5 minute block mean values from the ASCII data in hawaii.xyg, run

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

SEE ALSO

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

COPYRIGHT

       2018, P. Wessel, W. H. F. Smith, R. Scharroo, J. Luis, and F. Wobbe