blockmean(1gmt)
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 ] [ -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][=|+][/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 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 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]. If your
weights are actually uncertainties (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.
-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.
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 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
2016, P. Wessel, W. H. F. Smith, R. Scharroo, J. Luis, and F. Wobbe
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