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PROGRAM:

NAME

blockmean - Block average (x, y, z) data tables by L2 norm

SYNOPSIS

blockmean [ table ] increment region [ ] [ [p] ] [ [m|n|s|w] ] [ [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

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]
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. 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.

--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 mean values from the ASCII data in hawaii.xyg, run

gmt blockmean hawaii.xyg -R198/208/18/25 -I5m > hawaii_5x5.xyg

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