NAME
nearneighbor - A "Nearest neighbor" gridding algorithm
SYNOPSIS
nearneighbor [ xyzfile(s) ] -Gout_grdfile
-Ix_inc[m|c][/y_inc[m|c]] -Nsectors
-Rwest/east/south/north[r] -Ssearch_radius[m|c|k|K] [
-Eempty ] [ -F ] [ -H[nrec] ] [ -Lflag ] [ -V ] [ -W ] [ -:
] [ -bi[s][n] ]
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 cen-
tered 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 sec-
tors for a given node. Nodes that fail this test are
assigned the value NaN (but see -E). [Default is qua-
drant 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 spac-
ing; 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].
OPTIONS
-E Set the value assigned to empty nodes [NaN].
-F Force pixel registration. [Default is grid registra-
tion].
-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 condi-
tions]
-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)].
-W Input data have a 4th column containing observation
point weights. These are multiplied with the geometri-
cal 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].
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, and set empty nodes
to -9999, try
nearneighbor seaMARCII_bathy.lon_lat_z -R242/244/-22/-20
-I.5m -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
SEE ALSO
blockmean(l), blockmedian(l), blockmode(l), gmt(l),
surface(l), triangulate(l)