nearneighbor - A "Nearest neighbor" gridding algorithm
nearneighbor [
xyzfile(s) ]
-Gout_grdfile
-Ix_inc[
m|c][/
y_inc[
m|c]]
-Nsectors -Rwest/east/south/north[
r]
-S search_radius[
m|c|k|K] [
-Eempty ] [
-F ] [
-H[
nrec] ] [
-L[
flag] ] [
-V
] [
-W ] [
-: ] [
-bi[
s][
n] ]
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) = 1.0 / (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 grdfile.
- -I
-
x_inc [and optionally y_inc] is the grid
spacing. Append m to indicate minutes or c to indicate
seconds.
- -N
- The circular area centered on each node is divided into
several sectors. Average values will only be computed if there is at least
one value inside each of the sectors for a given node. Nodes that fail
this test are assigned the value NaN (but see -E). [Default is
quadrant search, i.e., sectors = 4]. Note that only the nearest
value per sector enters into the averaging, not all values inside the
circle.
- -R
-
west, east, south, and north specify the
Region of interest. To specify boundaries in degrees and minutes [and
seconds], use the dd:mm[:ss] format. Append r if lower left and
upper right map coordinates are given instead of wesn.
- -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 indicated km (implies -R -I are in
degrees); use uppercase K if distances should be calculated using
great circles [ k uses flat Earth].
- -E
- Set the value assigned to empty nodes [NaN].
- -F
- Force pixel registration. [Default is grid
registration].
- -H
- Input file(s) has Header record(s). Number of header
records can be changed by editing your .gmtdefaults file. If used,
GMT default is 1 header record. 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].
If no flag is given, it is assumed that the x column contains longitudes,
which may differ from the region in -R by [multiples of] 360
degrees [Default assumes no periodicity].
- -V
- Selects verbose mode, which will send progress reports to
stderr [Default runs "silently"].
- -:
- Toggles between (longitude,latitude) and
(latitude,longitude) input/output. [Default is (longitude,latitude)].
Applies to geographic coordinates only.
- -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 double]. Append n for the number of columns in the
binary file(s). [Default is 3 (or 4 if -W is set) columns].
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, and set empty nodes to
-9999, try
nearneighbor seaMARCII_bathy.lon_lat_z
-R242/244/-22/-20
-I.5
m
-E-9999
-Gbathymetry.grd
-S5k
-N8
To make a global gridded 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
empty nodes to NaN, try
nearneighbor geoid.xyz
-R0/360/-90/90
-I1
-Lg
-Ggeoid.grd
-S20K
-N4
blockmean(1gmt),
blockmedian(1gmt),
blockmode(1gmt),
gmt(1gmt),
surface(1gmt),
triangulate(1gmt)