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**-N**\ [**f**\ \|\ **q**\ \|\ **s**\ \|\ *nx/ny*][\ **+a**\ \|\ [\ **+d**\ \|\ **h**\ \|\ **l**][\ **+e**\ \|\ **n**\ \|\ **m**][\ **+t**\ *width*][**+w**\ [*suffix*]][\ **+z**\ [**p**]]
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.
**-N**\ *nx/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 **+t**\ *width*. 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 *ingrid*\ \_phase.\ *ext*.
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