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grdfft - Do mathematical operations on grids in the wavenumber (or frequency) domain


grdfft ingrid [ ingrid2 ] outfile [ azimuth ] [ zlevel ] [ [scale|g] ] [ [r|x|y][w[k]] ] [
[r|x|y]params ] [ [scale|g] ] [
[f|q|s|nx/ny][+a|d|h|l][+e|n|m][+twidth][+w[suffix]][+z[p]] ] [ scale ] [ [level] ] [ -fg

Note: No space is allowed between the option flag and the associated arguments.


grdfft will take the 2-D forward Fast Fourier Transform and perform one or more
mathematical operations in the frequency domain before transforming back to the space
domain. An option is provided to scale the data before writing the new values to an output
file. The horizontal dimensions of the grid are assumed to be in meters. Geographical
grids may be used by specifying the -fg option that scales degrees to meters. If you have
grids with dimensions in km, you could change this to meters using grdedit or scale the
output with grdmath.


ingrid 2-D binary grid file to be operated on. (See GRID FILE FORMATS below). For
cross-spectral operations, also give the second grid file ingrd2.

Specify the name of the output grid file or the 1-D spectrum table (see -E). (See


Take the directional derivative in the azimuth direction measured in degrees CW
from north.

Upward (for zlevel > 0) or downward (for zlevel < 0) continue the field zlevel

Differentiate the field, i.e., take d(field)/dz. This is equivalent to multiplying
by kr in the frequency domain (kr is radial wave number). Append a scale to
multiply by (kr * scale) instead. Alternatively, append g to indicate that your
data are geoid heights in meters and output should be gravity anomalies in mGal.
[Default is no scale].

Estimate power spectrum in the radial direction [r]. Place x or y immediately after
-E to compute the spectrum in the x or y direction instead. No grid file is
created. If one grid is given then f (i.e., frequency or wave number), power[f],
and 1 standard deviation in power[f] are written to the file set by -G [stdout]. If
two grids are given we write f and 8 quantities: Xpower[f], Ypower[f], coherent
power[f], noise power[f], phase[f], admittance[f], gain[f], coherency[f]. Each
quantity is followed by its own 1-std dev error estimate, hence the output is 17
columns wide. Append w to write wavelength instead of frequency. If your grid is
geographic you may further append k to scale wavelengths from meter [Default] to

Filter the data. Place x or y immediately after -F to filter x or y direction only;
default is isotropic [r]. Choose between a cosine-tapered band-pass, a Gaussian
band-pass filter, or a Butterworth band-pass filter.

Specify four wavelengths lc/lp/hp/hc in correct units (see -fg) to design a
bandpass filter: wavelengths greater than lc or less than hc will be cut,
wavelengths greater than lp and less than hp will be passed, and wavelengths
in between will be cosine-tapered. E.g., -F1000000/250000/50000/10000 -fg
will bandpass, cutting wavelengths > 1000 km and < 10 km, passing
wavelengths between 250 km and 50 km. To make a highpass or lowpass filter,
give hyphens (-) for hp/hc or lc/lp. E.g., -Fx-/-/50/10 will lowpass x,
passing wavelengths > 50 and rejecting wavelengths < 10. -Fy1000/250/-/-
will highpass y, passing wavelengths < 250 and rejecting wavelengths > 1000.

Gaussian band-pass:
Append lo/hi, the two wavelengths in correct units (see -fg) to design a
bandpass filter. At the given wavelengths the Gaussian filter weights will
be 0.5. To make a highpass or lowpass filter, give a hyphen (-) for the hi
or lo wavelength, respectively. E.g., -F-/30 will lowpass the data using a
Gaussian filter with half-weight at 30, while -F400/- will highpass the

Butterworth band-pass:
Append lo/hi/order, the two wavelengths in correct units (see -fg) and the
filter order (an integer) to design a bandpass filter. At the given
wavelengths the Butterworth filter weights will be 0.5. To make a highpass
or lowpass filter, give a hyphen (-) for the hi or lo wavelength,
respectively. E.g., -F-/30/2 will lowpass the data using a 2nd-order
Butterworth filter, with half-weight at 30, while -F400/-/2 will highpass
the data.

Integrate the field, i.e., compute integral_over_z (field * dz). This is
equivalent to divide by kr in the frequency domain (kr is radial wave number).
Append a scale to divide by (kr * scale) instead. Alternatively, append g to
indicate that your data set is gravity anomalies in mGal and output should be geoid
heights in meters. [Default is no scale].

Choose or inquire about suitable grid dimensions for FFT and set optional
parameters. Control the FFT dimension:
-Nf will force the FFT to use the actual dimensions of the data.

-Nq will inQuire about more suitable dimensions, report those, then continue.

-Ns will present a list of optional dimensions, then exit.

-Nnx/ny will do FFT on array size nx/ny (must be >= grid file size). Default
chooses dimensions >= data which optimize speed and accuracy of FFT. If FFT
dimensions > grid file dimensions, data are extended and tapered to zero.

Control detrending of data: Append modifiers for removing a linear trend:
+d: Detrend data, i.e. remove best-fitting linear trend [Default].

+a: Only remove mean value.

+h: Only remove mid value, i.e. 0.5 * (max + min).

+l: Leave data alone.

Control extension and tapering of data: Use modifiers to control how the extension
and tapering are to be performed:
+e extends the grid by imposing edge-point symmetry [Default],

+m extends the grid by imposing edge mirror symmetry

+n turns off data extension.

Tapering is performed from the data edge to the FFT grid edge [100%]. Change
this percentage via +twidth. When +n is in effect, the tapering is applied
instead to the data margins as no extension is available [0%].

Control writing of temporary results: For detailed investigation you can write the
intermediate grid being passed to the forward FFT; this is likely to have been
detrended, extended by point-symmetry along all edges, and tapered. Append
+w[suffix] from which output file name(s) will be created (i.e., ingrid_prefix.ext)
[tapered], where ext is your file extension. Finally, you may save the complex grid
produced by the forward FFT by appending +z. By default we write the real and
imaginary components to ingrid_real.ext and ingrid_imag.ext. Append p to save
instead the polar form of magnitude and phase to files ingrid_mag.ext and

Multiply each element by scale in the space domain (after the frequency domain
operations). [Default is 1.0].

-V[level] (more ...)
Select verbosity level [c].

-fg Geographic grids (dimensions of longitude, latitude) will be converted to meters
via a "Flat Earth" approximation using the current ellipsoid parameters.

-^ or just -
Print a short message about the syntax of the command, then exits (NOTE: on Windows
use just -).

-+ or just +
Print an extensive usage (help) message, including the explanation of any
module-specific option (but not the GMT common options), then exits.

-? or no arguments
Print a complete usage (help) message, including the explanation of options, then

Print GMT version and exit.

Print full path to GMT share directory and exit.


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 1- or 2-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. In case
the two characters id is not provided, as in =/scale than a id=nf is assumed. When
reading grids, the format is generally automatically recognized. If not, the same suffix
can be added to input grid file names. See grdconvert and Section grid-file-format 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
grdconvert and Sections modifiers-for-CF and grid-file-format of the GMT Technical
Reference and Cookbook for more information, particularly on how to read splices of 3-,
4-, or 5-dimensional grids.


If the grid does not have meter as the horizontal unit, append +uunit to the input file
name to convert from the specified unit to meter. If your grid is geographic, convert
distances to meters by supplying -fg instead.


netCDF COARDS grids will automatically be recognized as geographic. For other grids
geographical grids were you want to convert degrees into meters, select -fg. If the data
are close to either pole, you should consider projecting the grid file onto a rectangular
coordinate system using grdproject


To upward continue the sea-level magnetic anomalies in the file mag_0.nc to a level 800 m
above sealevel:

gmt grdfft mag_0.nc -C800 -V -Gmag_800.nc

To transform geoid heights in m (geoid.nc) on a geographical grid to free-air gravity
anomalies in mGal:

gmt grdfft geoid.nc -Dg -V -Ggrav.nc

To transform gravity anomalies in mGal (faa.nc) to deflections of the vertical (in
micro-radians) in the 038 direction, we must first integrate gravity to get geoid, then
take the directional derivative, and finally scale radians to micro-radians:

gmt grdfft faa.nc -Ig -A38 -S1e6 -V -Gdefl_38.nc

Second vertical derivatives of gravity anomalies are related to the curvature of the
field. We can compute these as mGal/m^2 by differentiating twice:

gmt grdfft gravity.nc -D -D -V -Ggrav_2nd_derivative.nc

To compute cross-spectral estimates for co-registered bathymetry and gravity grids, and
report result as functions of wavelengths in km, try

gmt grdfft bathymetry.nc gravity.grd -Ewk -fg -V > cross_spectra.txt

To examine the pre-FFT grid after detrending, point-symmetry reflection, and tapering has
been applied, as well as saving the real and imaginary components of the raw spectrum of
the data in topo.nc, try

gmt grdfft topo.nc -N+w+z -fg -V

You can now make plots of the data in topo_taper.nc, topo_real.nc, and topo_imag.nc.

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