1BLOCKMEAN(1) Generic Mapping Tools BLOCKMEAN(1)
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6 blockmean - filter to block average (x,y,z) data by L2 norm
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9 blockmean [ xyz[w]file(s) ] -Ixinc[unit][=|+][/yinc[unit][=|+]]
10 -Rxmin/xmax/ymin/ymax[r] [ -C ] [ -E ] [ -F ] [ -H[i][nrec] ] [ -S[w|z]
11 ] [ -V ] [ -W[io] ] [ -:[i|o] ] [ -b[i|o][s|S|d|D[ncol]|c[var1/...]] ]
12 [ -f[i|o]colinfo ]
13
15 blockmean 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 mean position and value for every non-
18 empty block in a grid region defined by the -R and -I arguments. Either
19 blockmean, blockmedian, or blockmode should be used as a pre-processor
20 before running surface to avoid aliasing short wavelengths. These rou‐
21 tines are also generally useful for decimating or averaging (x,y,z)
22 data. You can modify the precision of the output format by editing the
23 D_FORMAT parameter in your .gmtdefaults4 file, or you may choose binary
24 input and/or output using single or double precision storage.
25 xyz[w]file(s)
27 3 [or 4] column ASCII file(s) [or binary, see -b] holding
28 (x,y,z[,w]) data values. [w] is an optional weight for the
29 data. If no file is specified, blockmean will read from stan‐
30 dard input.
31 -I x_inc [and optionally y_inc] is the grid spacing. Optionally,
33 append a suffix modifier. Geographical (degrees) coordinates:
34 Append m to indicate arc minutes or c to indicate arc seconds.
35 If one of the units e, k, i, or n is appended instead, the
36 increment is assumed to be given in meter, km, miles, or nauti‐
37 cal miles, respectively, and will be converted to the equivalent
38 degrees longitude at the middle latitude of the region (the con‐
39 version depends on ELLIPSOID). If /y_inc is given but set to 0
40 it will be reset equal to x_inc; otherwise it will be converted
41 to degrees latitude. All coordinates: If = is appended then the
42 corresponding max x (east) or y (north) may be slightly adjusted
43 to fit exactly the given increment [by default the increment may
44 be adjusted slightly to fit the given domain]. Finally, instead
45 of giving an increment you may specify the number of nodes
46 desired by appending + to the supplied integer argument; the
47 increment is then recalculated from the number of nodes and the
48 domain. The resulting increment value depends on whether you
49 have selected a gridline-registered or pixel-registered grid;
50 see Appendix B for details. Note: if -Rgrdfile is used then
51 grid spacing has already been initialized; use -I to override
52 the values.
53 -R xmin, xmax, ymin, and ymax specify the Region of interest. For
55 geographic regions, these limits correspond to west, east,
56 south, and north and you may specify them in decimal degrees or
57 in [+-]dd:mm[:ss.xxx][W|E|S|N] format. Append r if lower left
58 and upper right map coordinates are given instead of w/e/s/n.
59 The two shorthands -Rg and -Rd stand for global domain (0/360
60 and -180/+180 in longitude respectively, with -90/+90 in lati‐
61 tude). Alternatively, specify the name of an existing grid file
62 and the -R settings (and grid spacing, if applicable) are copied
63 from the grid. For calendar time coordinates you may either
64 give (a) relative time (relative to the selected TIME_EPOCH and
65 in the selected TIME_UNIT; append t to -JX|x), or (b) absolute
66 time of the form [date]T[clock] (append T to -JX|x). At least
67 one of date and clock must be present; the T is always required.
68 The date string must be of the form [-]yyyy[-mm[-dd]] (Gregorian
69 calendar) or yyyy[-Www[-d]] (ISO week calendar), while the clock
70 string must be of the form hh:mm:ss[.xxx]. The use of delim‐
71 iters and their type and positions must be exactly as indicated
72 (however, input, output and plot formats are customizable; see
73 gmtdefaults).
74
76 -C Use the center of the block as the output location [Default uses
77 the mean location].
78 -E Provide Extended report which includes s (the standard deviation
80 of the mean), l, the lowest value, and h, the high value for
81 each block. Output order becomes x,y,z,s,l,h[,w]. [Default out‐
82 puts x,y,z[,w]. See -W for w output.
83 -F Force pixel node registration [Default is gridline registra‐
85 tion]. (Node registrations are defined in GMT Cookbook Appendix
86 B on grid file formats.) Each block is the locus of points
87 nearest the grid value location. For example, with
88 -R10/15/10/15 and and -I1: with the -F option 10 <= (x,y) < 11
89 is one of 25 blocks; without it 9.5 <= (x,y) < 10.5 is one of 36
90 blocks.
91 -H Input file(s) has header record(s). If used, the default number
93 of header records is N_HEADER_RECS. Use -Hi if only input data
94 should have header records [Default will write out header
95 records if the input data have them]. Blank lines and lines
96 starting with # are always skipped. Not used with binary data.
97 -S Use -Sz to report the sum of all z-values inside a block, or -Sw
99 to report the sum of weights [Default reports mean value]. If
100 -Sw is selected and no weights are supplied (i.e., no -W given),
101 then the weight sum will equal the number of points inside each
102 block.
103 -V Selects verbose mode, which will send progress reports to stderr
105 [Default runs "silently"].
106 -W Weighted modifier[s]. Unweighted input and output has 3 columns
108 x,y,z; Weighted i/o has 4 columns x,y,z,w. Weights can be used
109 in input to construct weighted mean values in blocks. Weight
110 sums can be reported in output for later combining several runs,
111 etc. Use -W for weighted i/o, -Wi for weighted inputonly, -Wo
112 for weighted output only. [Default uses unweighted i/o].
113 -: Toggles between (longitude,latitude) and (latitude,longitude)
115 input and/or output. [Default is (longitude,latitude)]. Append
116 i to select input only or o to select output only. [Default
117 affects both].
118 -bi Selects binary input. Append s for single precision [Default is
120 d (double)]. Uppercase S or D will force byte-swapping.
121 Optionally, append ncol, the number of columns in your binary
122 input file if it exceeds the columns needed by the program. Or
123 append c if the input file is netCDF. Optionally, append
124 var1/var2/... to specify the variables to be read. [Default is
125 3 (or 4 if -Wi is set)].
126 -bo Selects binary output. Append s for single precision [Default
128 is d (double)]. Uppercase S or D will force byte-swapping.
129 Optionally, append ncol, the number of desired columns in your
130 binary output file. [Default is 3 (or 4 if -Wo is set)]. -E
131 adds 3 additional columns.
132 -f Special formatting of input and/or output columns (time or geo‐
134 graphical data). Specify i or o to make this apply only to
135 input or output [Default applies to both]. Give one or more
136 columns (or column ranges) separated by commas. Append T (abso‐
137 lute calendar time), t (relative time in chosen TIME_UNIT since
138 TIME_EPOCH), x (longitude), y (latitude), or f (floating point)
139 to each column or column range item. Shorthand -f[i|o]g means
140 -f[i|o]0x,1y (geographic coordinates).
141
143 The ASCII output formats of numerical data are controlled by parameters
144 in your .gmtdefaults4 file. Longitude and latitude are formatted
145 according to OUTPUT_DEGREE_FORMAT, whereas other values are formatted
146 according to D_FORMAT. Be aware that the format in effect can lead to
147 loss of precision in the output, which can lead to various problems
148 downstream. If you find the output is not written with enough preci‐
149 sion, consider switching to binary output (-bo if available) or specify
150 more decimals using the D_FORMAT setting.
151
153 To find 5 by 5 minute block mean values from the ASCII data in
154 hawaii.xyg, run
155 blockmean hawaii.xyg -R198/208/18/25 -I5m > hawaii_5x5.xyg
157
159 blockmedian(1), blockmode(1), gmtdefaults(1), GMT(1), nearneighbor(1),
160 surface(1), triangulate(1)
161GMT 4.5.6 10 Mar 2011 BLOCKMEAN(1)