GRDFFT(l)																 GRDFFT(l)

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 hori-
       zontal 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 grd-
       math.
	       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 low-
	      pass filter, give hyphens (-) for hp/hc or lc/lp. E.g., -Fx-/-/50/10 will lowpass X, passing wavelengths > 50  and  rejecting  wave-
	      lengths < 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 dimen-
	      sions >= 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  densi-
	      ties 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 differenti-
       ating 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)).

SEE ALSO

       gmt(1gmt), grdedit(1gmt), grdmath(1gmt), grdproject(1gmt)

								    1 Jan 2004								 GRDFFT(l)