GRDFILTER(1) Generic Mapping Tools GRDFILTER(1)NAMEgrdfilter - Filter a 2-D grid file in the space (or time) domain
SYNOPSISgrdfilter input_file -Ddistance_flag -F<filtertype><width>[mode] -Gout‐
put_file [ -Ixinc[unit][=|+][/yinc[unit][=|+]] ] [ -Ni|p|r ] [
-Rwest/east/south/north[r] ] [ -T ] [ -V ] [ -f[i|o]colinfo ]
DESCRIPTIONgrdfilter 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.
input_file
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 dis‐
tance 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-convolu‐
tion filters. Append the filter code followed by the full diam‐
eter 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 ini‐
tial 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).
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.]
-N Determine how NaN-values in the input grid affects the filtered
outout: Append i to ignore all NaNs in the calculation of fil‐
tered 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 fil‐
tered 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 registra‐
tion 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 geo‐
graphical 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 (abso‐
lute 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 FILE FORMATS
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 facili‐
tates 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.
GEOGRAPHICAL AND TIME COORDINATES
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.
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 calcula‐
tions, you need:
grdfilter north_pacific_dbdb5.grd -G filtered_pacific.grd -Fm 600 -D 4
-R 150/250/10/40 -I 0.5 -V
LIMITATIONS
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 geo‐
graphic 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 spa‐
tial 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).
SEE ALSOGMT(1), grdfft(1)img2mercgrd(1)GMT 4.5.14 1 Nov 2015 GRDFILTER(1)
