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       grdfft - Perform mathematical operations on grdfiles in the frequency domain


       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 ]


       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.

              2-D binary grd file to be operated on.

       -G     Specify the name of the output grd file.


       -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

       -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

       -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

       -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


       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)).


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

                                            1 Jan 2004                                  GRDFFT(l)