BLOCKMEDIAN

Section: Misc. Reference Manual Pages (l)
Updated: DATE
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NAME

blockmedian - filter to block average (x,y,z) data by L1 norm.

SYNOPSIS

blockmedian [ xyz[w]file(s) ] -Ix_inc[m|c][/y_inc[m]] -Rwest/east/south/north[r] [ -H ] [ -N ] [ -V ] [ -W[io] ] [ -: ] [ -b[i|o][d] ]

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 or blockmedian 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 .gmtdefaults 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 block size. Append m to indicate minutes or c to indicate seconds. #include "explain_-R.txt"

OPTIONS

#include "explain_-H.txt" Not used with binary data.

-N: Offsets the grid registration of the boxes. Use pixel registration instead of grid registration. By default, the centers of the blocks coincide with the defined lattice nodes; each block contains all points nearest to a lattice node. The -N option will offset the blocks so their edges coincide with the lattice lines. For example, suppose the arguments are -R10/15/10/15 and -I1 This defines a 6 x 6 block array; the lower left block will average data within 9.5 <= (x,y) < 10.5. If the same lattice is given with the -N option, the block array is 5 x 5 and the lower left block will average the data within 10 <= (x,y) < 11. The default registration is normally used for preprocessing and gridding map data. #include "explain_-V.txt"
-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 median 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] #include "explain_-t.txt"
-b: Selects binary input and output mode. Append i or o to select single precision binary input or output only. [Default is ASCII]. Append d to indicate double precision for binary data (if both binary input and output are selected they will have the same precision).

EXAMPLES

To find 5 by 5 minute block medians from the double precision binary data in hawaii_b.xyg and output an ASCII table, try

blockmedian hawaii_b.xyg -R198/208/18/25 -I5m -bid > hawaii_5x5.xyg

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Time: 07:11:24 GMT, January 07, 2025