1GRDFILTER(1) Generic Mapping Tools GRDFILTER(1)
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6 grdfilter - Filter a 2-D grid file in the space (or time) domain
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9 grdfilter input_file -Ddistance_flag -F<filtertype><width>[mode] -Gout‐
10 put_file [ -Ixinc[unit][=|+][/yinc[unit][=|+]] ] [ -Ni|p|r ] [
11 -Rwest/east/south/north[r] ] [ -T ] [ -V ] [ -f[i|o]colinfo ]
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14 grdfilter will filter a .grd file in the time domain using one of the
15 selected convolution or non-convolution isotropic filters and compute
16 distances using Cartesian or Spherical geometries. The output .grd
17 file can optionally be generated as a subOPT(R)egion of the input
18 and/or with a new -Increment. In this way, one may have "extra space"
19 in the input data so that the edges will not be used and the output can
20 be within one-half- width of the input edges. If the filter is low-
21 pass, then the output may be less frequently sampled than the input.
22 input_file
24 The grid file of points to be filtered. (See GRID FILE FORMATS
25 below).
26 -D Distance flag tells how grid (x,y) relates to filter width as
28 follows:
29 flag = 0: grid (x,y) same units as width, Cartesian distances.
31 flag = 1: grid (x,y) in degrees, width in kilometers, Cartesian
32 distances.
33 flag = 2: grid (x,y) in degrees, width in km, dx scaled by
34 cos(middle y), Cartesian distances.
35 The above options are fastest because they allow weight matrix
37 to be computed only once. The next three options are slower
38 because they recompute weights for each latitude.
39 flag = 3: grid (x,y) in degrees, width in km, dx scaled by
41 cosine(y), Cartesian distance calculation.
42 flag = 4: grid (x,y) in degrees, width in km, Spherical dis‐
43 tance calculation.
44 flag = 5: grid (x,y) in Mercator -Jm1 img units, width in km,
45 Spherical distance calculation.
46 -F Sets the filter type. Choose among convolution and non-convolu‐
48 tion filters. Append the filter code followed by the full diam‐
49 eter width. Available convolution filters are:
50 (b) Boxcar: All weights are equal.
51 (c) Cosine Arch: Weights follow a cosine arch curve.
52 (g) Gaussian: Weights are given by the Gaussian function, where
53 width is 6 times the conventional Gaussian sigma.
54 Non-convolution filters are:
55 (m) Median: Returns median value.
56 (p) Maximum likelihood probability (a mode estimator): Return
57 modal value. If more than one mode is found we return their
58 average value. Append - or + to the filter width if you rather
59 want to return the smallest or largest of the modal values.
60 (l) Lower: Return the minimum of all values.
61 (L) Lower: Return minimum of all positive values only.
62 (u) Upper: Return maximum of all values.
63 (U) Upper: Return maximum or all negative values only.
64 In the case of L|U it is possible that no data passes the ini‐
65 tial sign test; in that case the filter will return 0.0.
66 -G output_file is the output grid file of the filter. (See GRID
68 FILE FORMATS below).
69
71 -I x_inc [and optionally y_inc] is the output Increment. Append m
72 to indicate minutes, or c to indicate seconds. If the new
73 x_inc, y_inc are NOT integer multiples of the old ones (in the
74 input data), filtering will be considerably slower. [Default:
75 Same as input.]
76 -N Determine how NaN-values in the input grid affects the filtered
78 outout: Append i to ignore all NaNs in the calculation of fil‐
79 tered value [Default], r is same as i except if the input node
80 was NaN then the output node will be set to NaN (only applies if
81 both grids are coregistered), and p which will force the fil‐
82 tered value to be NaN if any grid-nodes with NaN-values are
83 found inside the filter circle.
84 -R west, east, south, and north defines the Region of the output
86 points. [Default: Same as input.]
87 -T Toggle the node registration for the output grid so as to become
89 the opposite of the input grid [Default gives the same registra‐
90 tion as the input grid].
91 -V Selects verbose mode, which will send progress reports to stderr
93 [Default runs "silently"].
94 -f Special formatting of input and/or output columns (time or geo‐
96 graphical data). Specify i or o to make this apply only to
97 input or output [Default applies to both]. Give one or more
98 columns (or column ranges) separated by commas. Append T (abso‐
99 lute calendar time), t (relative time in chosen TIME_UNIT since
100 TIME_EPOCH), x (longitude), y (latitude), or f (floating point)
101 to each column or column range item. Shorthand -f[i|o]g means
102 -f[i|o]0x,1y (geographic coordinates).
103
105 By default GMT writes out grid as single precision floats in a COARDS-
106 complaint netCDF file format. However, GMT is able to produce grid
107 files in many other commonly used grid file formats and also facili‐
108 tates so called "packing" of grids, writing out floating point data as
109 2- or 4-byte integers. To specify the precision, scale and offset, the
110 user should add the suffix =id[/scale/offset[/nan]], where id is a two-
111 letter identifier of the grid type and precision, and scale and offset
112 are optional scale factor and offset to be applied to all grid values,
113 and nan is the value used to indicate missing data. When reading
114 grids, the format is generally automatically recognized. If not, the
115 same suffix can be added to input grid file names. See grdreformat(1)
116 and Section 4.17 of the GMT Technical Reference and Cookbook for more
117 information.
118 When reading a netCDF file that contains multiple grids, GMT will read,
120 by default, the first 2-dimensional grid that can find in that file. To
121 coax GMT into reading another multi-dimensional variable in the grid
122 file, append ?varname to the file name, where varname is the name of
123 the variable. Note that you may need to escape the special meaning of ?
124 in your shell program by putting a backslash in front of it, or by
125 placing the filename and suffix between quotes or double quotes. The
126 ?varname suffix can also be used for output grids to specify a variable
127 name different from the default: "z". See grdreformat(1) and Section
128 4.18 of the GMT Technical Reference and Cookbook for more information,
129 particularly on how to read splices of 3-, 4-, or 5-dimensional grids.
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132 When the output grid type is netCDF, the coordinates will be labeled
133 "longitude", "latitude", or "time" based on the attributes of the input
134 data or grid (if any) or on the -f or -R options. For example, both
135 -f0x -f1t and -R90w/90e/0t/3t will result in a longitude/time grid.
136 When the x, y, or z coordinate is time, it will be stored in the grid
137 as relative time since epoch as specified by TIME_UNIT and TIME_EPOCH
138 in the .gmtdefaults file or on the command line. In addition, the unit
139 attribute of the time variable will indicate both this unit and epoch.
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142 Suppose that north_pacific_dbdb5.grd is a file of 5 minute bathymetry
143 from 140E to 260E and 0N to 50N, and you want to find the medians of
144 values within a 300km radius (600km full width) of the output points,
145 which you choose to be from 150E to 250E and 10N to 40N, and you want
146 the output values every 0.5 degree. Using spherical distance calcula‐
147 tions, you need:
148 grdfilter north_pacific_dbdb5.grd -Gfiltered_pacific.grd -Fm600 -D4
150 -R150/250/10/40 -I0.5 -V
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153 When working with geographic (lat, lon) grids, all three convolution
154 filters (boxcar, cosine arch, and gaussian) will properly normalize the
155 filter weights for the variation in gridbox size with latitude, and
156 correctly determine which nodes are needed for the convolution when the
157 filter "circle" crosses a periodic (0-360) boundary or contains a geo‐
158 graphic pole. However, the spatial filters, such as median and mode
159 filters, do not use weights and thus should only be used on Cartesian
160 grids (or at very low latitudes) only. If you want to apply such spa‐
161 tial filters you should project your data to an equal-area projection
162 and run grdfilter on the resulting Cartesian grid.
163 To use the -D5 option the input Mercator grid must be created by
164 img2mercgrd using the -C option so the origin of the y-values is the
165 Equator (i.e., x = y = 0 correspond to lon = lat = 0).
166
168 GMT(1), grdfft(1) img2mercgrd(1)
169GMT 4.5.6 10 Mar 2011 GRDFILTER(1)