NAME
blockmode - filter to block average (x,y,z) data by mode estimation.
SYNOPSIS
blockmode [ xyz[w]file(s) ] -Ix_inc[m|c][/y_inc[m|c]]
-Rwest/east/south/north[r] [ -C ] [ -F ] [ -H[nrec] ] [ -Q ] [ -V ] [
-W[io] ] [ -: ] [ -bi[s][n] ] [ -bo[s][n] ] [ -f[i|o]colinfo ]
DESCRIPTION
blockmode 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 mode estimates of position and value for
every non-empty block in a grid region defined by the -R and -I argu-
ments. 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 D_FORMAT parameter in your .gmtdefaults4 file, or
you may choose binary input and/or output using single or double preci-
sion 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, blockmode will read from standard
input.
-I x_inc [and optionally y_inc] is the grid spacing. Append m to
indicate minutes or c to indicate seconds.
-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 wesn. The
two shorthands -Rg -Rd stand for global domain (0/360 or
-180/+180 in longitude respectively, with -90/+90 in latitude).
For calendar time coordinates you may either give relative time
(relative to the selected TIME_EPOCH and in the selected
TIME_UNIT; append t to -JX|x), or 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 as indicated (however, input/output
and plotting formats are flexible).
OPTIONS
-C Use the center of the block as the output location [Default uses
the modal xy location (but see -Q)]. -C overrides -Q.
-F Block centers have pixel registration. [Default: grid registra-
tion.] (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 -R10/15/10/15 and and
-I1: 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). Number of header records can
be changed by editing your .gmtdefaults4 file. If used, GMT
default is 1 header record. Use -Hi if only input data should
have header records [Default will write out header records if
the input data have them]. Not used with binary data.
-Q (Quicker) Finds mode z and mean (x, y) [Default finds mode x,
mode y, mode z].
-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 modal 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].
-: 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
double]. Append n for the number of columns in the binary
file(s).
[Default is 3 (or 4 if -W is set) columns].
-bo Selects binary output. Append s for single precision [Default is
double]. Append n for the number of columns in the binary
file(s).
-f Special formatting of input and output columns (time or geo-
graphical data) Specify i(nput) or o(utput) [Default is both
input and output]. Give one or more columns (or column ranges)
separated by commas. Append T (Absolute calendar time), t (time
relative to chosen TIME_EPOCH), x (longitude), y (latitude), g
(geographic coordinate), or f (floating point) to each column or
column range item.
EXAMPLES
To find 5 by 5 minute block mode estimates from the double precision
binary data in hawaii_b.xyg and output an ASCII table, run:
blockmode hawaii_b.xyg -R198/208/18/25 -I5m -bi3 > hawaii_5x5.xyg
SEE ALSO
blockmean(l), blockmedian(l), gmt(l), gmtdefaults(l), nearneighbor(l),
surface(l), triangulate(l)
GMT4.0 1 Oct 2004 BLOCKMODE(l)
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