Ubuntu Manpage: grdfft - Perform mathematical operations on grdfiles in the frequency domain

NAME

       grdfft - Perform mathematical operations on grdfiles in the frequency domain

SYNOPSIS

       grdfft  in_grdfile -Gout_grdfile [ -Aazimuth ] [ -Czlevel ] [ -D[scale|g] ] [ -E[x|y][w] ]
       [ -F[x|y]lc/lp/hp/hc ] [ -I[scale|g] ] [  -L  ]  [  -M  ]  [  -Nstuff  ]  [  -Sscale  ]  [
       -Tte/rl/rm/rw/ri ] [ -V ]

DESCRIPTION

       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 grdfiles are assumed to be in meters.  Geographical
       grids  may  be used by specifying the -M option that scales degrees to meters. If you have
       grdfiles with dimensions in km, you could change this to meters using grdedit or scale the
       output with grdmath.
               No  space between the option flag and the associated arguments. Use upper case for
       the option flags and lower case for modifiers.

       in_grdfile
              2-D binary grd file to be operated on.

       -G     Specify the name of the output grd file.

OPTIONS

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

-C Upward (for zlevel > 0) or downward (for zlevel < 0) continue the field zlevel
meters.

-D 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].

-E Estimate power spectrum in the radial direction. Place x or y immediately after -E
to compute the spectrum in the x or y direction instead. No grdfile is created; f
(i.e., frequency or wave number), power[f], and 1 standard deviation in power[f]
are written to stdout. Append w to write wavelength instead of frequency.

-F Filter the data. Place x or y immediately after -F to filter x or y direction only;
default is isotropic. Specify four wavelengths in correct units (see -M) 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 -M 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.

-I 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].

-L Leave trend alone. By default, a linear trend will be removed prior to the
transform.

-M Map units. Choose this option if your grdfile is a geographical grid and you want
to convert degrees into meters. If the data are close to either pole, you should
consider projecting the grdfile onto a rectangular coordinate system using
grdproject.

-N Choose or inquire about suitable grid dimensions for FFT. -Nf will force the FFT
to use the dimensions of the data. -Nq will inQuire about more suitable
dimensions. -Nnx/ny will do FFT on array size nx/ny (Must be >= grdfile size).
Default chooses dimensions >= data which optimize speed, accuracy of FFT. If FFT
dimensions > grdfile dimensions, data are extended and tapered to zero.

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

-T Compute the isostatic compensation from the topography load (input grdfile) on an
elastic plate of thickness te. Also append densities for load, mantle, water, and
infill in SI units. If te == 0 then the Airy response is returned. -T implicitly
sets -L.

-V Selects verbose mode, which will send progress reports to stderr [Default runs
"silently"].

EXAMPLES

       To upward continue the sea-level magnetic anomalies in the file mag_0.grd to a level 800 m
       above sealevel, try

       grdfft mag_0.grd -C800 -V -Gmag_800.grd

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

       grdfft geoid.grd -Dg -M -V -Ggrav.grd

       To transform gravity anomalies in mGal (faa.grd) 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:

       grdfft faa.grd -Ig -A38 -S1e6 -V -Gdefl_38.grd

       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:

       grdfft gravity.grd -D -D -V -Ggrav_2nd_derivative.grd

       The first order gravity anomaly (in mGal) due to the compensating surface  caused  by  the
       topography load topo.grd (in m) on a 20 km thick elastic plate, assumed to be 4 km beneath
       the observation level can be computed as

       grdfft topo.grd -T20000/2800/3330/1030/2300 -S0.022 -C4000 -Gcomp_faa.grd

       where 0.022 is the scale needed for the first term in Parker's expansion for  '  computing
       gravity from topography (= 2 * PI * G * (rhom - rhol)).

NAME

SYNOPSIS

DESCRIPTION

OPTIONS

EXAMPLES

SEE ALSO