blockmedian(1) GMT blockmedian(1)
NAME
blockmedian - Block average (x, y, z) data tables by L1 norm
SYNOPSIS
blockmedian [ table ]
-Iincrement
-Rregion [ -C ] [ -E[b] ] [ -Er|s[-] ] [ -Q ] [ -Tquantile ] [
-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
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 wave-
lengths. These routines are also generally useful for decimating or
averaging (x,y,z) data. You can modify the precision of the output for-
mat 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, nau-
tical mile or US survey foot, respectively, and will be con-
verted to the equivalent degrees longitude at the middle lati-
tude 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 coordi-
nates: 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 recalcu-
lated from the number of nodes and the domain. The resulting
increment value depends on whether you have selected a grid-
line-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 a|)
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, blockme-
dian 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 val-
ues; 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 a|)
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 median values for each block.
Weight sums can be reported in output for later combining sev-
eral 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 a|)
Select native binary input. [Default is 3 (or 4 if -Wi is set)].
-bo[ncols][type] (more a|)
Select native binary output. [Default is 3 (or 4 if -Wo is
set)]. -E adds 3 additional columns.
-d[i|o]nodata (more a|)
Replace input columns that equal nodata with NaN and do the
reverse on output.
-e[~]^<i>apattern^<i>a | -e[~]/regexp/[i] (more a|)
Only accept data records that match the given pattern.
-f[i|o]colinfo (more a|)
Specify data types of input and/or output columns.
-h[i|o][n][+c][+d][+rremark][+rtitle] (more a|)
Skip or produce header record(s).
-icols[+l][+sscale][+ooffset][,^<i>a|] (more a|)
Select input columns and transformations (0 is first column).
-ocols[,a|] (more a|)
Select output columns (0 is first column).
-r (more a|)
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 a|)
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 explana-
tion 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 FOR-
MAT_DATE_OUT and FORMAT_CLOCK_OUT, whereas general floating point val-
ues are formatted according to FORMAT_FLOAT_OUT. Be aware that the for-
mat 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 set-
ting.
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% quan-
tiles. 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(1), blockmode(1), gmt(1), gmt.conf(5), greenspline(1),
nearneighbor(1), surface(1), sphtriangulate(1), triangulate(1)
COPYRIGHT
2017, P. Wessel, W. H. F. Smith, R. Scharroo, J. Luis, and F. Wobbe
5.4.2 Jun 24, 2017 blockmedian(1)
gmt5 5.4.2 - Generated Wed Jun 28 15:12:59 CDT 2017