1BLOCKMEDIAN(1) Generic Mapping Tools BLOCKMEDIAN(1)
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6 blockmedian - filter to block average (x,y,z) data by L1 norm.
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9 blockmedian [ xyz[w]file(s) ] -Ixinc[unit][=|+][/yinc[unit][=|+]]
10 -Rxmin/xmax/ymin/ymax[r] [ -C ] [ -E ] [ -F ] [ -H[i][nrec] ] [ -Q ] [
11 -Tquartile ] [ -V ] [ -W[io] ] [ -:[i|o] ] [
12 -b[i|o][s|S|d|D[ncol]|c[var1/...]] ] [ -f[i|o]colinfo ]
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15 blockmedian reads arbitrarily located (x,y,z) triples [or optionally
16 weighted quadruples (x,y,z,w)] from standard input [or xyz[w]file(s)]
17 and writes to standard output a median position and value for every
18 non-empty block in a grid region defined by the -R and -I arguments.
19 Either blockmean, blockmedian, or blockmode should be used as a pre-
20 processor before running surface to avoid aliasing short wavelengths.
21 These routines are also generally useful for decimating or averaging
22 (x,y,z) data. You can modify the precision of the output format by
23 editing the D_FORMAT parameter in your .gmtdefaults4 file, or you may
24 choose binary input and/or output using single or double precision
25 storage.
26 xyz[w]file(s)
28 3 [or 4] column ASCII file(s) [or binary, see -b] holding
29 (x,y,z[,w]) data values. [w] is an optional weight for the
30 data. If no file is specified, blockmedian will read from stan‐
31 dard input.
32 -I x_inc [and optionally y_inc] is the grid spacing. Optionally,
34 append a suffix modifier. Geographical (degrees) coordinates:
35 Append m to indicate arc minutes or c to indicate arc seconds.
36 If one of the units e, k, i, or n is appended instead, the
37 increment is assumed to be given in meter, km, miles, or nauti‐
38 cal miles, respectively, and will be converted to the equivalent
39 degrees longitude at the middle latitude of the region (the con‐
40 version depends on ELLIPSOID). If /y_inc is given but set to 0
41 it will be reset equal to x_inc; otherwise it will be converted
42 to degrees latitude. All coordinates: If = is appended then the
43 corresponding max x (east) or y (north) may be slightly adjusted
44 to fit exactly the given increment [by default the increment may
45 be adjusted slightly to fit the given domain]. Finally, instead
46 of giving an increment you may specify the number of nodes
47 desired by appending + to the supplied integer argument; the
48 increment is then recalculated from the number of nodes and the
49 domain. The resulting increment value depends on whether you
50 have selected a gridline-registered or pixel-registered grid;
51 see Appendix B for details.
52 -R xmin, xmax, ymin, and ymax specify the Region of interest. For
54 geographic regions, these limits correspond to west, east,
55 south, and north and you may specify them in decimal degrees or
56 in [+-]dd:mm[:ss.xxx][W|E|S|N] format. Append r if lower left
57 and upper right map coordinates are given instead of w/e/s/n.
58 The two shorthands -Rg and -Rd stand for global domain (0/360
59 and -180/+180 in longitude respectively, with -90/+90 in lati‐
60 tude). For calendar time coordinates you may either give (a)
61 relative time (relative to the selected TIME_EPOCH and in the
62 selected TIME_UNIT; append t to -JX|x), or (b) absolute time of
63 the form [date]T[clock] (append T to -JX|x). At least one of
64 date and clock must be present; the T is always required. The
65 date string must be of the form [-]yyyy[-mm[-dd]] (Gregorian
66 calendar) or yyyy[-Www[-d]] (ISO week calendar), while the clock
67 string must be of the form hh:mm:ss[.xxx]. The use of delim‐
68 iters and their type and positions must be exactly as indicated
69 (however, input, output and plot formats are customizable; see
70 gmtdefaults).
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73 -C Use the center of the block as the output location [Default uses
74 the median location (but see -Q)]. -C overrides -Q.
75 -E Provide Extended report which includes s (the L1 scale of the
77 median), l, the lowest value, and h, the high value for each
78 block. Output order becomes x,y,z,s,l,h[,w]. [Default outputs
79 x,y,z[,w]. See -W for w output.
80 -F Force pixel node registration [Default is gridline registra‐
82 tion]. (Node registrations are defined in GMT Cookbook Appendix
83 B on grid file formats.) Each block is the locus of points
84 nearest the grid value location. For example, with
85 -R10/15/10/15 and and -I1: with the -F option 10 <= (x,y) < 11
86 is one of 25 blocks; without it 9.5 <= (x,y) < 10.5 is one of 36
87 blocks.
88 -H Input file(s) has Header record(s). Number of header records
90 can be changed by editing your .gmtdefaults4 file. If used, GMT
91 default is 1 header record. Use -Hi if only input data should
92 have header records [Default will write out header records if
93 the input data have them]. Blank lines and lines starting with #
94 are always skipped. Not used with binary data.
95 -Q (Quicker) Finds median z and (x, y) at that z [Default finds
97 median x, median y, median z].
98 -T Sets the quartile of the distribution to be returned [Default is
100 0.5 which returns the median z]. Here, 0 < quartile < 1.
101 -V Selects verbose mode, which will send progress reports to stderr
103 [Default runs "silently"].
104 -W Weighted modifier[s]. Unweighted input and output has 3 columns
106 x,y,z; Weighted i/o has 4 columns x,y,z,w. Weights can be used
107 in input to construct weighted mean values in blocks. Weight
108 sums can be reported in output for later combining several runs,
109 etc. Use -W for weighted i/o, -Wi for weighted inputonly, -Wo
110 for weighted output only. [Default uses unweighted i/o].
111 -: Toggles between (longitude,latitude) and (latitude,longitude)
113 input and/or output. [Default is (longitude,latitude)]. Append
114 i to select input only or o to select output only. [Default
115 affects both].
116 -bi Selects binary input. Append s for single precision [Default is
118 d (double)]. Uppercase S or D will force byte-swapping.
119 Optionally, append ncol, the number of columns in your binary
120 input file if it exceeds the columns needed by the program. Or
121 append c if the input file is netCDF. Optionally, append
122 var1/var2/... to specify the variables to be read. [Default is
123 3 (or 4 if -Wi is set)].
124 -bo Selects binary output. Append s for single precision [Default
126 is d (double)]. Uppercase S or D will force byte-swapping.
127 Optionally, append ncol, the number of desired columns in your
128 binary output file. [Default is 3 (or 4 if -Wo is set)]. -E
129 adds 3 additional columns.
130 -f Special formatting of input and/or output columns (time or geo‐
132 graphical data). Specify i or o to make this apply only to
133 input or output [Default applies to both]. Give one or more
134 columns (or column ranges) separated by commas. Append T (abso‐
135 lute calendar time), t (relative time in chosen TIME_UNIT since
136 TIME_EPOCH), x (longitude), y (latitude), or f (floating point)
137 to each column or column range item. Shorthand -f[i|o]g means
138 -f[i|o]0x,1y (geographic coordinates).
139
141 The ASCII output formats of numerical data are controlled by parameters
142 in your .gmtdefaults4 file. Longitude and latitude are formatted
143 according to OUTPUT_DEGREE_FORMAT, whereas other values are formatted
144 according to D_FORMAT. Be aware that the format in effect can lead to
145 loss of precision in the output, which can lead to various problems
146 downstream. If you find the output is not written with enough preci‐
147 sion, consider switching to binary output (-bo if available) or specify
148 more decimals using the D_FORMAT setting.
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151 To find 5 by 5 minute block medians from the double precision binary
152 data in hawaii_b.xyg and output an ASCII table, run
153 blockmedian hawaii_b.xyg -R198/208/18/25 -I5m -bi3 > hawaii_5x5.xyg
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157 blockmean(1), blockmode(1), GMT(1), gmtdefaults(1), nearneighbor(1),
158 surface(1), triangulate(1)
159GMT 4.3.1 15 May 2008 BLOCKMEDIAN(1)