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NEARNEIGHBOR(1) Generic Mapping Tools NEARNEIGHBOR(1)
NAME
nearneighbor - A "Nearest neighbor" gridding algorithm
SYNOPSIS
nearneighbor [ xyzfile(s) ] -Gout_grdfile
-Ixinc[unit][=|*][/yinc[unit][=|+]] -Nsectors[/min_sectors]
-Rwest/east/south/north[r] -Ssearch_radius[m|c|k|K] [ -Eempty ] [ -F ]
[ -H[i][nrec] ] [ -Lflag ] [ -V ] [ -W ] [ -:[i|o] ] [
-bi[s|S|d|D[ncol]|c[var1/...]] ] [ -fcolinfo ]
DESCRIPTION
nearneighbor reads arbitrarily located (x,y,z[,w]) triples
[quadruplets] from standard input [or xyzfile(s)] and uses a nearest
neighbor algorithm to assign an average value to each node that have
one or more points within a radius centered on the node. The average
value is computed as a weighted mean of the nearest point from each
sector inside the search radius. The weighting function used is w(r) =
1 / (1 + d ^ 2), where d = 3 * r / search_radius and r is distance from
the node. This weight is modulated by the observation points' weights
[if supplied].
xyzfile(s)
3 [or 4, see -W] column ASCII file(s) [or binary, see -b]
holding (x,y,z[,w]) data values. If no file is specified,
nearneighbor will read from standard input.
-G Give the name of the output grid file.
-I x_inc [and optionally y_inc] is the grid spacing. Optionally,
append a suffix modifier. Geographical (degrees) coordinates:
Append m to indicate arc minutes or c to indicate arc seconds.
If one of the units e, k, i, or n is appended instead, the
increment is assumed to be given in meter, km, miles, or
nautical miles, respectively, and will be converted to the
equivalent degrees longitude at the middle latitude of the
region (the conversion depends on ELLIPSOID). If /y_inc is
given but set to 0 it will be reset equal to x_inc; otherwise it
will be converted to degrees latitude. All coordinates: If = is
appended then the corresponding max x (east) or y (north) may be
slightly adjusted to fit exactly the given increment [by default
the increment may be adjusted slightly to fit the given domain].
Finally, instead of giving an increment you may specify the
number of nodes desired by appending * to the supplied integer
argument; the increment is then recalculated from the number of
nodes and the domain. The resulting increment value depends on
whether you have selected a gridline-registered or pixel-
registered grid; see Appendix B for details. Note: if -Rgrdfile
is used then grid spacing has already been initialized; use -I
to override the values.
-N The circular area centered on each node is divided into sectors
sectors. Average values will only be computed if there is at
least one value inside each of at least min_sectors of these
sectors for a given node. Nodes that fail this test are
assigned the value NaN (but see -E). If min_sectors is omitted
it is set to be at least 50% of sectors (i.e., rounded up to
next integer). [Default is a quadrant search with 100%
coverage, i.e., sectors = min_sectors = 4]. Note that only the
nearest value per sector enters into the averaging; the more
distant points are ignored.
-R xmin, xmax, ymin, and ymax specify the Region of interest. For
geographic regions, these limits correspond to west, east,
south, and north and you may specify them in decimal degrees or
in [+-]dd:mm[:ss.xxx][W|E|S|N] format. Append r if lower left
and upper right map coordinates are given instead of w/e/s/n.
The two shorthands -Rg and -Rd stand for global domain (0/360
and -180/+180 in longitude respectively, with -90/+90 in
latitude). Alternatively, specify the name of an existing grid
file and the -R settings (and grid spacing, if applicable) are
copied from the grid. For calendar time coordinates you may
either give (a) relative time (relative to the selected
TIME_EPOCH and in the selected TIME_UNIT; append t to -JX|x), or
(b) absolute time of the form [date]T[clock] (append T to
-JX|x). At least one of date and clock must be present; the T
is always required. The date string must be of the form
[-]yyyy[-mm[-dd]] (Gregorian calendar) or yyyy[-Www[-d]] (ISO
week calendar), while the clock string must be of the form
hh:mm:ss[.xxx]. The use of delimiters and their type and
positions must be exactly as indicated (however, input, output
and plot formats are customizable; see gmtdefaults).
-S Sets the search_radius in same units as the grid spacing; append
m to indicate minutes or c to indicate seconds. Append k to
indicate km (implies -R and -I are in degrees, and we will use a
fast flat Earth approximation to calculate distance). For more
accuracy, use uppercase K if distances should be calculated
along geodesics. However, if the current ELLIPSOID is spherical
then great circle calculations are used.
OPTIONS
-E Set the value assigned to empty nodes [NaN].
-F Force pixel node registration [Default is gridline
registration]. (Node registrations are defined in GMT Cookbook
Appendix B on grid file formats.)
-H Input file(s) has header record(s). If used, the default number
of header records is N_HEADER_RECS. Use -Hi if only input data
should have header records [Default will write out header
records if the input data have them]. Blank lines and lines
starting with # are always skipped. Not used with binary data.
-L Boundary condition flag may be x or y or xy indicating data is
periodic in range of x or y or both set by -R, or flag may be g
indicating geographical conditions (x and y are lon and lat).
[Default is no boundary conditions].
-V Selects verbose mode, which will send progress reports to stderr
[Default runs "silently"].
-: Toggles between (longitude,latitude) and (latitude,longitude)
input and/or output. [Default is (longitude,latitude)]. Append
i to select input only or o to select output only. [Default
affects both].
-W Input data have a 4th column containing observation point
weights. These are multiplied with the geometrical weight
factor to determine the actual weights used in the calculations.
-bi Selects binary input. Append s for single precision [Default is
d (double)]. Uppercase S or D will force byte-swapping.
Optionally, append ncol, the number of columns in your binary
input file if it exceeds the columns needed by the program. Or
append c if the input file is netCDF. Optionally, append
var1/var2/... to specify the variables to be read. [Default is
3 (or 4 if -W is set) columns].
-f Special formatting of input and/or output columns (time or
geographical data). Specify i or o to make this apply only to
input or output [Default applies to both]. Give one or more
columns (or column ranges) separated by commas. Append T
(absolute calendar time), t (relative time in chosen TIME_UNIT
since TIME_EPOCH), x (longitude), y (latitude), or f (floating
point) to each column or column range item. Shorthand -f[i|o]g
means -f[i|o]0x,1y (geographic coordinates).
GRID VALUES PRECISION
Regardless of the precision of the input data, GMT programs that create
grid files will internally hold the grids in 4-byte floating point
arrays. This is done to conserve memory and furthermore most if not
all real data can be stored using 4-byte floating point values. Data
with higher precision (i.e., double precision values) will lose that
precision once GMT operates on the grid or writes out new grids. To
limit loss of precision when processing data you should always consider
normalizing the data prior to processing.
EXAMPLES
To create a gridded data set from the file seaMARCII_bathy.lon_lat_z
using a 0.5 min grid, a 5 km search radius, using an octant search with
100% sector coverage, and set empty nodes to -9999:
nearneighbor seaMARCII_bathy.lon_lat_z -R 242/244/-22/-20 -I 0.5m
-E-9999 -G bathymetry.grd -S 5k -N 8/8
To make a global grid file from the data in geoid.xyz using a 1 degree
grid, a 200 km search radius, spherical distances, using an quadrant
search, and set nodes to NaN only when fewer than two quadrants contain
at least one value:
nearneighbor geoid.xyz -R 0/360/-90/90 -I 1 -L g -G geoid.grd -S 200K
-N 4
SEE ALSO
blockmean(1), blockmedian(1), blockmode(1), GMT(1), surface(1),
triangulate(1)
GMT 4.5.14 1 Nov 2015 NEARNEIGHBOR(1)