NEARNEIGHBOR(1) Generic Mapping Tools NEARNEIGHBOR(1)NAMEnearneighbor - A "Nearest neighbor" gridding algorithm
SYNOPSISnearneighbor [ 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 ]
DESCRIPTIONnearneighbor reads arbitrarily located (x,y,z[,w]) triples [quadru‐
plets] 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 com‐
puted 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 sup‐
plied].
xyzfile(s)
3 [or 4, see -W] column ASCII file(s) [or binary, see -b] hold‐
ing (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 nauti‐
cal miles, respectively, and will be converted to the equivalent
degrees longitude at the middle latitude of the region (the con‐
version 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% cover‐
age, i.e., sectors = min_sectors = 4]. Note that only the near‐
est 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 lati‐
tude). 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 delim‐
iters 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 registra‐
tion]. (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 fac‐
tor 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 geo‐
graphical 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 (abso‐
lute 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 ALSOblockmean(1), blockmedian(1), blockmode(1), GMT(1), surface(1), trian‐
gulate(1)GMT 4.5.14 1 Nov 2015 NEARNEIGHBOR(1)
