gmt-fork/doc/rst/source/explain_fft.rst_

**-N**\ [**a**\|\ **f**\|\ **m**\|\ **r**\|\ **s**\|\ *nx/ny*][**+a**\|\ **d**\|\ **h**\|\ **l**][**+e**\|\ **n**\|\ **m**][**+t**\ *width*][**+v**][**+w**\ [*suffix*]][**+z**\ [**p**]]
    Choose or inquire about suitable grid dimensions for FFT and set
    optional parameters. Control the FFT dimension:

    	**-Na** lets the FFT select dimensions yielding the most accurate result.

        **-Nf** will force the FFT to use the actual dimensions of the data.

    	**-Nm** lets the FFT select dimensions using the least work memory.

    	**-Nr** lets the FFT select dimensions yielding the most rapid calculation.

        **-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 messages being reported:
        **+v** will report suitable dimensions during processing.

    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*.