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[k] [ -Eempty ] [ -
F ] [ -H ] [ -V ] [ -W ] [ -: ] [ -b[d] ]
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 all points inside the search radius. The
weighting function used is w(r) = 1.0 / (1 + d * d), 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 com-
puted 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].
-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 k to indicated km (implies -R -I are
in degrees).
OPTIONS
-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.
-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.
-b Selects binary input mode [Default is ASCII]. Append
d to indicate double precision for binary data [Default
is single].
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
SEE ALSO
blockmean, blockmedian, gmt, surface, triangulate
REFERENCES
Wessel, P., and W. H. F. Smith, 1995, The Generic Mapping
Tools (GMT) version 3.0 Technical Reference & Cookbook,
SOEST/NOAA.
Wessel, P., and W. H. F. Smith, 1995, New Version of the
Generic Mapping Tools Released, EOS Trans. AGU, 76, p. 329.
Wessel, P., and W. H. F. Smith, 1995, New Version of the
Generic Mapping Tools Released,
http://www.agu.org/eos_elec/95154e.html, Copyright 1995 by
the American Geophysical Union.
Wessel, P., and W. H. F. Smith, 1991, Free Software Helps
Map and Display Data, EOS Trans. AGU, 72, p. 441.