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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
standard 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
nautical miles, respectively, and will be converted to the
equivalent degrees longitude at the middle latitude of the
region (the conversion 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
latitude). 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 delimiters 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
registration]. (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
geographical 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
(absolute 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
precision, 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)