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Manual Reference Pages  -  GRDFILTER (1)


grdfilter - Filter a 2-D grid file in the space (or time) domain


Grid File Formats
Geographical And Time Coordinates


grdfilter input_file -Ddistance_flag -F<filtertype><width>[mode] -Goutput_file [ -Ixinc[unit][=|+][/yinc[unit][=|+]] ] [ -Ni|p|r ] [ -Rwest/east/south/north[r] ] [ -T ] [ -V ] [ -f[i|o]colinfo ]


grdfilter will filter a .grd file in the time domain using one of the selected convolution or non-convolution isotropic filters and compute distances using Cartesian or Spherical geometries. The output .grd file can optionally be generated as a subOPT(R)egion of the input and/or with a new -I ncrement. 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.
  The grid file of points to be filtered. (See GRID FILE FORMATS below).
-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 three options are slower because they recompute weights for each latitude.

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.
flag = 5: grid (x,y) in Mercator -Jm1 img units, width in km, Spherical distance calculation.

-F Sets the filter type. Choose among convolution and non-convolution filters. Append the filter code followed by the full diameter width. Available convolution filters are:
(b) Boxcar: All weights are equal.
(c) Cosine Arch: Weights follow a cosine arch curve.
(g) Gaussian: Weights are given by the Gaussian function, where width is 6 times the conventional Gaussian sigma.
Non-convolution filters are:
(m) Median: Returns median value.
(p) Maximum likelihood probability (a mode estimator): Return modal value. If more than one mode is found we return their average value. Append - or + to the filter width if you rather want to return the smallest or largest of the modal values.
(l) Lower: Return the minimum of all values.
(L) Lower: Return minimum of all positive values only.
(u) Upper: Return maximum of all values.
(U) Upper: Return maximum or all negative values only.
In the case of L|U it is possible that no data passes the initial sign test; in that case the filter will return 0.0.
-G output_file is the output grid file of the filter. (See GRID FILE FORMATS below).


-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.]
-N Determine how NaN-values in the input grid affects the filtered outout: Append i to ignore all NaNs in the calculation of filtered value [Default], r is same as i except if the input node was NaN then the output node will be set to NaN (only applies if both grids are coregistered), and p which will force the filtered value to be NaN if any grid-nodes with NaN-values are found inside the filter circle.
-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"].
-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).


By default GMT writes out grid as single precision floats in a COARDS-complaint netCDF file format. However, GMT is able to produce grid files in many other commonly used grid file formats and also facilitates so called "packing" of grids, writing out floating point data as 2- or 4-byte integers. To specify the precision, scale and offset, the user should add the suffix =id[/scale/offset[/nan]], where id is a two-letter identifier of the grid type and precision, and scale and offset are optional scale factor and offset to be applied to all grid values, and nan is the value used to indicate missing data. When reading grids, the format is generally automatically recognized. If not, the same suffix can be added to input grid file names. See grdreformat(1) and Section 4.17 of the GMT Technical Reference and Cookbook for more information.

When reading a netCDF file that contains multiple grids, GMT will read, by default, the first 2-dimensional grid that can find in that file. To coax GMT into reading another multi-dimensional variable in the grid file, append ?varname to the file name, where varname is the name of the variable. Note that you may need to escape the special meaning of ? in your shell program by putting a backslash in front of it, or by placing the filename and suffix between quotes or double quotes. The ?varname suffix can also be used for output grids to specify a variable name different from the default: "z". See grdreformat(1) and Section 4.18 of the GMT Technical Reference and Cookbook for more information, particularly on how to read splices of 3-, 4-, or 5-dimensional grids.


When the output grid type is netCDF, the coordinates will be labeled "longitude", "latitude", or "time" based on the attributes of the input data or grid (if any) or on the -f or -R options. For example, both -f0x -f1t and -R 90w/90e/0t/3t will result in a longitude/time grid. When the x, y, or z coordinate is time, it will be stored in the grid as relative time since epoch as specified by TIME_UNIT and TIME_EPOCH in the .gmtdefaults file or on the command line. In addition, the unit attribute of the time variable will indicate both this unit and epoch.


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 -G filtered_pacific.grd -Fm 600 -D 4 -R 150/250/10/40 -I 0.5 -V


When working with geographic (lat, lon) grids, all three convolution filters (boxcar, cosine arch, and gaussian) will properly normalize the filter weights for the variation in gridbox size with latitude, and correctly determine which nodes are needed for the convolution when the filter "circle" crosses a periodic (0-360) boundary or contains a geographic pole. However, the spatial filters, such as median and mode filters, do not use weights and thus should only be used on Cartesian grids (or at very low latitudes) only. If you want to apply such spatial filters you should project your data to an equal-area projection and run grdfilter on the resulting Cartesian grid.
To use the -D 5 option the input Mercator grid must be created by img2mercgrd using the -C option so the origin of the y-values is the Equator (i.e., x = y = 0 correspond to lon = lat = 0).


GMT(1), grdfft(1) img2mercgrd(1)
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GMT 4.5.14 GRDFILTER (1) 1 Nov 2015

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