grdfilter(1gmt) [debian man page]
GRDFILTER(l) GRDFILTER(l) NAME
grdfilter - Filter a .grd file in the Time domain SYNOPSIS
grdfilter input_file.grd -Dflag -F<type><width> -Goutput_file.grd [ -Ix_inc[m|c][/y_inc[m|c]] ] [ -Rwest/east/south/north[r] ] [ -T ] [ -V ] DESCRIPTION
grdfilter will filter a .grd file in the time domain using a boxcar, cosine arch, gaussian, median, or mode filter and computing distances using Cartesian or Spherical geometries. The output .grd file can optionally be generated as a sub-Region of the input and/or with a new -Increment. In this way, one may have "extra space" in the input data so that the edges will not be used and the output can be within one- half- width of the input edges. If the filter is low-pass, then the output may be less frequently sampled than the input. input_file.grd The file of points to be filtered. -D Distance flag tells how grid (x,y) relates to filter width as follows: flag = 0: grid (x,y) same units as width, Cartesian distances. flag = 1: grid (x,y) in degrees, width in kilometers, Cartesian distances. flag = 2: grid (x,y) in degrees, width in km, dx scaled by cos(middle y), Cartesian distances. The above options are fastest because they allow weight matrix to be computed only once. The next two options are slower because they recompute weights for each East-West scan line. flag = 3: grid (x,y) in degrees, width in km, dx scaled by cosine(y), Cartesian distance calculation. flag = 4: grid (x,y) in degrees, width in km, Spherical distance calculation. -F Choose one only of bcgmp for (b)oxcar, (c)osine Arch, (g)aussian, (m)edian, or maximum likelihood (p)robability (a mode estimator) filter and specify full width. -G output_file.grd is the output of the filter. OPTIONS
-I x_inc [and optionally y_inc] is the output Increment. Append m to indicate minutes, or c to indicate seconds. If the new x_inc, y_inc are NOT integer multiples of the old ones (in the input data), filtering will be considerably slower. [Default: Same as input.] -R west, east, south, and north defines the Region of the output points. [Default: Same as input.] -T Toggle the node registration for the output grid so as to become the opposite of the input grid [Default gives the same registration as the input grid]. -V Selects verbose mode, which will send progress reports to stderr [Default runs "silently"]. EXAMPLES
Suppose that north_pacific_dbdb5.grd is a file of 5 minute bathymetry from 140E to 260E and 0N to 50N, and you want to find the medians of values within a 300km radius (600km full width) of the output points, which you choose to be from 150E to 250E and 10N to 40N, and you want the output values every 0.5 degree. Using spherical distance calculations, you need: grdfilter north_pacific_dbdb5.grd -Gfiltered_pacific.grd -Fm600 -D4 -R150/250/10/40 -I0.5 -V SEE ALSO
gmt(1gmt), grdfft(1gmt) 1 Jan 2004 GRDFILTER(l)
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NEARNEIGHBOR(l) NEARNEIGHBOR(l) 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] ] [ -L[flag] ] [ -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 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 indi- cated 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 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 indi- cating 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 geo- graphic coordinates only. -W Input data have a 4th column containing observation point weights. These are multiplied with the geometrical weight factor to deter- mine 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 quad- rant search, and set empty nodes to NaN, try nearneighbor geoid.xyz -R0/360/-90/90 -I1 -Lg -Ggeoid.grd -S20K -N4 SEE ALSO
blockmean(1gmt), blockmedian(1gmt), blockmode(1gmt), gmt(1gmt), surface(1gmt), triangulate(1gmt) 1 Jan 2004 NEARNEIGHBOR(l)