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Manual Reference Pages  -  NEARNEIGHBOR (1)

NAME

nearneighbor - A "Nearest neighbor" gridding algorithm

CONTENTS

Synopsis
Description
Options
Grid Values Precision
Examples

SYNOPSIS

nearneighbor [ 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 ]

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 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 / (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 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 nautical miles, respectively, and will be converted to the equivalent degrees longitude at the middle latitude of the region (the conversion 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% 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.
-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 latitude). 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 delimiters 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 registration]. (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 factor 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 geographical 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 (absolute 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 ALSO

blockmean(1), blockmedian(1), blockmode(1), GMT(1), surface(1), triangulate(1)
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GMT 4.5.14 NEARNEIGHBOR (1) 1 Nov 2015

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