1NEARNEIGHBOR(1) Generic Mapping Tools NEARNEIGHBOR(1)
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6 nearneighbor - A "Nearest neighbor" gridding algorithm
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9 nearneighbor [ xyzfile(s) ] -Gout_grdfile
10 -Ixinc[unit][=|+][/yinc[unit][=|+]] -Nsectors[/min_sectors]
11 -Rwest/east/south/north[r] -Ssearch_radius[m|c|k|K] [ -Eempty ] [ -F ]
12 [ -H[i][nrec] ] [ -Lflag ] [ -V ] [ -W ] [ -:[i|o] ] [
13 -bi[s|S|d|D[ncol]|c[var1/...]] ] [ -fcolinfo ]
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16 nearneighbor reads arbitrarily located (x,y,z[,w]) triples [quadru‐
17 plets] from standard input [or xyzfile(s)] and uses a nearest neighbor
18 algorithm to assign an average value to each node that have one or more
19 points within a radius centered on the node. The average value is com‐
20 puted as a weighted mean of the nearest point from each sector inside
21 the search radius. The weighting function used is w(r) = 1 / (1 + d ^
22 2), where d = 3 * r / search_radius and r is distance from the node.
23 This weight is modulated by the observation points' weights [if sup‐
24 plied].
25 xyzfile(s)
27 3 [or 4, see -W] column ASCII file(s) [or binary, see -b] hold‐
28 ing (x,y,z[,w]) data values. If no file is specified,
29 nearneighbor will read from standard input.
30 -G Give the name of the output grid file.
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 -N The circular area centered on each node is divided into sectors
54 sectors. Average values will only be computed if there is at
55 least one value inside at least min_sectors of the sectors for a
56 given node. Nodes that fail this test are assigned the value
57 NaN (but see -E). If min_sectors is omitted, each sector needs
58 to have at least one value inside it. [Default is quadrant
59 search, i.e., sectors = min_sectors = 4]. Note that only the
60 nearest value per sector enters into the averaging, not all val‐
61 ues inside the circle.
62 -R xmin, xmax, ymin, and ymax specify the Region of interest. For
64 geographic regions, these limits correspond to west, east,
65 south, and north and you may specify them in decimal degrees or
66 in [+-]dd:mm[:ss.xxx][W|E|S|N] format. Append r if lower left
67 and upper right map coordinates are given instead of w/e/s/n.
68 The two shorthands -Rg and -Rd stand for global domain (0/360
69 and -180/+180 in longitude respectively, with -90/+90 in lati‐
70 tude). For calendar time coordinates you may either give (a)
71 relative time (relative to the selected TIME_EPOCH and in the
72 selected TIME_UNIT; append t to -JX|x), or (b) absolute time of
73 the form [date]T[clock] (append T to -JX|x). At least one of
74 date and clock must be present; the T is always required. The
75 date string must be of the form [-]yyyy[-mm[-dd]] (Gregorian
76 calendar) or yyyy[-Www[-d]] (ISO week calendar), while the clock
77 string must be of the form hh:mm:ss[.xxx]. The use of delim‐
78 iters and their type and positions must be exactly as indicated
79 (however, input, output and plot formats are customizable; see
80 gmtdefaults).
81 -S Sets the search_radius in same units as the grid spacing; append
83 m to indicate minutes or c to indicate seconds. Append k to
84 indicate km (implies -R and -I are in degrees, and we will use a
85 fast flat Earth approximation to calculate distance). For more
86 accuracy, use uppercase K if distances should be calculated
87 along geodesics. However, if the current ELLIPSOID is set to
88 Sphere then spherical great circle calculations are used.
89
91 -E Set the value assigned to empty nodes [NaN].
92 -F Force pixel node registration [Default is gridline registra‐
94 tion]. (Node registrations are defined in GMT Cookbook Appendix
95 B on grid file formats.)
96 -H Input file(s) has Header record(s). Number of header records
98 can be changed by editing your .gmtdefaults4 file. If used, GMT
99 default is 1 header record. Use -Hi if only input data should
100 have header records [Default will write out header records if
101 the input data have them]. Blank lines and lines starting with #
102 are always skipped. Not used with binary data.
103 -L Boundary condition flag may be x or y or xy indicating data is
105 periodic in range of x or y or both set by -R, or flag may be g
106 indicating geographical conditions (x and y are lon and lat).
107 [Default is no boundary conditions].
108 -V Selects verbose mode, which will send progress reports to stderr
110 [Default runs "silently"].
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 -W Input data have a 4th column containing observation point
118 weights. These are multiplied with the geometrical weight fac‐
119 tor to determine the actual weights used in the calculations.
120 -bi Selects binary input. Append s for single precision [Default is
122 d (double)]. Uppercase S or D will force byte-swapping.
123 Optionally, append ncol, the number of columns in your binary
124 input file if it exceeds the columns needed by the program. Or
125 append c if the input file is netCDF. Optionally, append
126 var1/var2/... to specify the variables to be read. [Default is
127 3 (or 4 if -W is set) columns].
128 -f Special formatting of input and/or output columns (time or geo‐
130 graphical data). Specify i or o to make this apply only to
131 input or output [Default applies to both]. Give one or more
132 columns (or column ranges) separated by commas. Append T (abso‐
133 lute calendar time), t (relative time in chosen TIME_UNIT since
134 TIME_EPOCH), x (longitude), y (latitude), or f (floating point)
135 to each column or column range item. Shorthand -f[i|o]g means
136 -f[i|o]0x,1y (geographic coordinates).
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139 Regardless of the precision of the input data, GMT programs that create
140 gridded files will internally hold the grids in 4-byte floating point
141 arrays. This is done to conserve memory and futhermore most if not all
142 real data can be stored using 4-byte floating point values. Data with
143 higher precision (i.e., double precision values) will lose that preci‐
144 sion once GMT operates on the grid or writes out new grids. To limit
145 loss of precision when processing data you should always consider nor‐
146 malizing the data prior to processing.
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149 To create a gridded data set from the file seaMARCII_bathy.lon_lat_z
150 using a 0.5 min grid, a 5 km search radius, using an octant search, and
151 set empty nodes to -9999:
152 nearneighbor seaMARCII_bathy.lon_lat_z -R242/244/-22/-20 -I.5m -E-9999
154 -Gbathymetry.grd -S5k -N8
155 To make a global gridded file from the data in geoid.xyz using a 1
157 degree grid, a 200 km search radius, spherical distances, using an
158 quadrant search, and set nodes to NaN only when fewer than two quad‐
159 rants contain at least one value:
160 nearneighbor geoid.xyz -R0/360/-90/90 -I1 -Lg -Ggeoid.grd -S200K -N4/2
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164 blockmean(1), blockmedian(1), blockmode(1), GMT(1), surface(1), trian‐
165 gulate(1)
166GMT 4.3.1 15 May 2008 NEARNEIGHBOR(1)