Provided by: gmt-common_5.2.1+dfsg-3build1_all 
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
5.2.1 January 28, 2016 BLOCKMEDIAN(1gmt)