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


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.


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.


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.


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.

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

-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.

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].

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

Print GMT version and exit.

Print full path to GMT share directory and exit.


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.


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

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