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[b] ] [ -F ] [ -H[i][nrec] ] [ -Q ]
11 [ -Tquantile ] [ -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. Note: if -Rgrdfile is used then
52 grid spacing has already been initialized; use -I to override
53 the values.
54 -R xmin, xmax, ymin, and ymax specify the Region of interest. For
56 geographic regions, these limits correspond to west, east,
57 south, and north and you may specify them in decimal degrees or
58 in [+-]dd:mm[:ss.xxx][W|E|S|N] format. Append r if lower left
59 and upper right map coordinates are given instead of w/e/s/n.
60 The two shorthands -Rg and -Rd stand for global domain (0/360
61 and -180/+180 in longitude respectively, with -90/+90 in lati‐
62 tude). Alternatively, specify the name of an existing grid file
63 and the -R settings (and grid spacing, if applicable) are copied
64 from the grid. For calendar time coordinates you may either
65 give (a) relative time (relative to the selected TIME_EPOCH and
66 in the selected TIME_UNIT; append t to -JX|x), or (b) absolute
67 time of the form [date]T[clock] (append T to -JX|x). At least
68 one of date and clock must be present; the T is always required.
69 The date string must be of the form [-]yyyy[-mm[-dd]] (Gregorian
70 calendar) or yyyy[-Www[-d]] (ISO week calendar), while the clock
71 string must be of the form hh:mm:ss[.xxx]. The use of delim‐
72 iters and their type and positions must be exactly as indicated
73 (however, input, output and plot formats are customizable; see
74 gmtdefaults).
75
77 -C Use the center of the block as the output location [Default uses
78 the median x and median y as location (but see -Q)]..
79 -E Provide Extended report which includes s (the L1 scale of the
81 median), l, the lowest value, and h, the high value for each
82 block. Output order becomes x,y,z,s,l,h[,w]. [Default outputs
83 x,y,z[,w]. For box-and-whisker calculation, use -Eb which will
84 output x,y,z,l,q25,q75,h[,w], where q25 and q75 are the 25% and
85 75% quantiles, respectively. See -W for w output.
86 -F Force pixel node registration [Default is gridline registra‐
88 tion]. (Node registrations are defined in GMT Cookbook Appendix
89 B on grid file formats.) Each block is the locus of points
90 nearest the grid value location. For example, with
91 -R10/15/10/15 and and -I1: with the -F option 10 <= (x,y) < 11
92 is one of 25 blocks; without it 9.5 <= (x,y) < 10.5 is one of 36
93 blocks.
94 -H Input file(s) has header record(s). If used, the default number
96 of header records is N_HEADER_RECS. Use -Hi if only input data
97 should have header records [Default will write out header
98 records if the input data have them]. Blank lines and lines
99 starting with # are always skipped. Not used with binary data.
100 -Q (Quicker) Finds median z and (x, y) at that the median z
102 [Default finds median x, median y independent of z]. Also see
103 -C.
104 -T Sets the quantile of the distribution to be returned [Default is
106 0.5 which returns the median z]. Here, 0 < quantile < 1.
107 -V Selects verbose mode, which will send progress reports to stderr
109 [Default runs "silently"].
110 -W Weighted modifier[s]. Unweighted input and output has 3 columns
112 x,y,z; Weighted i/o has 4 columns x,y,z,w. Weights can be used
113 in input to construct weighted mean values in blocks. Weight
114 sums can be reported in output for later combining several runs,
115 etc. Use -W for weighted i/o, -Wi for weighted inputonly, -Wo
116 for weighted output only. [Default uses unweighted i/o].
117 -: Toggles between (longitude,latitude) and (latitude,longitude)
119 input and/or output. [Default is (longitude,latitude)]. Append
120 i to select input only or o to select output only. [Default
121 affects both].
122 -bi Selects binary input. Append s for single precision [Default is
124 d (double)]. Uppercase S or D will force byte-swapping.
125 Optionally, append ncol, the number of columns in your binary
126 input file if it exceeds the columns needed by the program. Or
127 append c if the input file is netCDF. Optionally, append
128 var1/var2/... to specify the variables to be read. [Default is
129 3 (or 4 if -Wi is set)].
130 -bo Selects binary output. Append s for single precision [Default
132 is d (double)]. Uppercase S or D will force byte-swapping.
133 Optionally, append ncol, the number of desired columns in your
134 binary output file. [Default is 3 (or 4 if -Wo is set)]. -E
135 adds 3 additional columns.
136 -f Special formatting of input and/or output columns (time or geo‐
138 graphical data). Specify i or o to make this apply only to
139 input or output [Default applies to both]. Give one or more
140 columns (or column ranges) separated by commas. Append T (abso‐
141 lute calendar time), t (relative time in chosen TIME_UNIT since
142 TIME_EPOCH), x (longitude), y (latitude), or f (floating point)
143 to each column or column range item. Shorthand -f[i|o]g means
144 -f[i|o]0x,1y (geographic coordinates).
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147 The ASCII output formats of numerical data are controlled by parameters
148 in your .gmtdefaults4 file. Longitude and latitude are formatted
149 according to OUTPUT_DEGREE_FORMAT, whereas other values are formatted
150 according to D_FORMAT. Be aware that the format in effect can lead to
151 loss of precision in the output, which can lead to various problems
152 downstream. If you find the output is not written with enough preci‐
153 sion, consider switching to binary output (-bo if available) or specify
154 more decimals using the D_FORMAT setting.
155
157 To find 5 by 5 minute block medians from the double precision binary
158 data in hawaii_b.xyg and output an ASCII table, run
159 blockmedian hawaii_b.xyg -R198/208/18/25 -I5m -bi3 > hawaii_5x5.xyg
161 To compute the shape of a data distribution per bin via a box-and-
163 whisker diagram we need the 0%, 25%, 50%, 75%, and 100% quantiles. To
164 do so on a global 5 by 5 degree basis from the ASCII table depths.xyz
165 and send output to an ASCII table, run
166 blockmedian depths.xyz -Rg -I5 -F -Eb > depths_5x5.txt
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170 blockmean(1), blockmode(1), GMT(1), gmtdefaults(1), nearneighbor(1),
171 surface(1), triangulate(1)
172GMT 4.5.6 10 Mar 2011 BLOCKMEDIAN(1)