nearneighbor(1) GMT nearneighbor(1)
NAME
nearneighbor - "Grid table data using a ""Nearest neighbor"" algorithm"
SYNOPSIS
nearneighbor [ table ] -Gout_grdfile
-Iincrement
-Nsectors[/min_sectors]
-Rregion
-Ssearch_radius[unit] [ -Eempty ] [ -V[level] ] [ -W ] [ -bibinary
] [ -dinodata ] [ -eregexp ] [ -fflags ] [ -hheaders ] [ -iflags ] [
-nflags ] [ -r ] [ -:[i|o] ]
Note: No space is allowed between the option flag and the associated
arguments.
DESCRIPTION
nearneighbor reads arbitrarily located (x,y,z[,w]) triples [quadru-
plets] from standard input [or table] and uses a nearest neighbor algo-
rithm 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 weights of the observation points [if
supplied].
REQUIRED ARGUMENTS
-Gout_grdfile
Give the name of the output grid file.
-Ixinc[unit][+e|n][/yinc[unit][+e|n]]
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 s to indicate arc seconds.
If one of the units e, f, k, M, n or u is appended instead, the
increment is assumed to be given in meter, foot, km, Mile, nau-
tical mile or US survey foot, respectively, and will be con-
verted to the equivalent degrees longitude at the middle lati-
tude of the region (the conversion depends on PROJ_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 coordi-
nates: If +e 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 +n to
the supplied integer argument; the increment is then recalcu-
lated from the number of nodes and the domain. The resulting
increment value depends on whether you have selected a grid-
line-registered or pixel-registered grid; see App-file-formats
for details. Note: if -Rgrdfile is used then the grid spacing
has already been initialized; use -I to override the values.
-Nsectors[/min_sectors]
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 the sec-
tors 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 inte-
ger). [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.
-Rxmin/xmax/ymin/ymax[+r][+uunit] (more a|)
Specify the region of interest.
-Ssearch_radius[unit]
Sets the search_radius that determines which data points are
considered close to a node. Append the distance unit (see
UNITS).
OPTIONAL ARGUMENTS
table 3 [or 4, see -W] column ASCII file(s) [or binary, see -bi] hold-
ing (x,y,z[,w]) data values. If no file is specified, nearneigh-
bor will read from standard input.
-Eempty
Set the value assigned to empty nodes [NaN].
-V[level] (more a|)
Select verbosity level [c].
-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[ncols][t] (more a|)
Select native binary input. [Default is 3 (or 4 if -W is set)
columns].
-dinodata (more a|)
Replace input columns that equal nodata with NaN.
-e[~]^<i>apattern^<i>a | -e[~]/regexp/[i] (more a|)
Only accept data records that match the given pattern.
-f[i|o]colinfo (more a|)
Specify data types of input and/or output columns.
-h[i|o][n][+c][+d][+rremark][+rtitle] (more a|)
Skip or produce header record(s).
-icols[+l][+sscale][+ooffset][,^<i>a|] (more a|)
Select input columns and transformations (0 is first column).
-n[b|c|l|n][+a][+bBC][+tthreshold]
Append +bBC to set any boundary conditions to be used, adding g
for geographic, p for periodic, or n for natural boundary condi-
tions. For the latter two you may append x or y to specify just
one direction, otherwise both are assumed. [Default is geo-
graphic if grid is geographic].
-r (more a|)
Set pixel node registration [gridline].
-:[i|o] (more a|)
Swap 1st and 2nd column on input and/or output.
-^ or just -
Print a short message about the syntax of the command, then
exits (NOTE: on Windows just use -).
-+ or just +
Print an extensive usage (help) message, including the explana-
tion of any module-specific option (but not the GMT common
options), then exits.
-? or no arguments
Print a complete usage (help) message, including the explanation
of all options, then exits.
UNITS
For map distance unit, append unit d for arc degree, m for arc minute,
and s for arc second, or e for meter [Default], f for foot, k for km, M
for statute mile, n for nautical mile, and u for US survey foot. By
default we compute such distances using a spherical approximation with
great circles. Prepend - to a distance (or the unit is no distance is
given) to perform aFlat Eartha calculations (quicker but less accurate)
or prepend + to perform exact geodesic calculations (slower but more
accurate).
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 preci-
sion once GMT operates on the grid or writes out new grids. To limit
loss of precision when processing data you should always consider nor-
malizing 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:
gmt nearneighbor seaMARCII_bathy.lon_lat_z -R242/244/-22/-20 -I0.5m \
-E-9999 -Gbathymetry.nc -S5k -N8/8
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:
gmt nearneighbor geoid.xyz -R0/360/-90/90 -I1 -Lg -Ggeoid.nc -S200k -N4
SEE ALSO
blockmean(1), blockmedian(1), blockmode(1), gmt(1), greenspline(1),
sphtriangulate(1), surface(1), triangulate(1)
COPYRIGHT
2017, P. Wessel, W. H. F. Smith, R. Scharroo, J. Luis, and F. Wobbe
5.4.2 Jun 24, 2017 nearneighbor(1)
gmt5 5.4.2 - Generated Thu Jun 29 14:52:15 CDT 2017