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**NAME**

grdfft - Perform mathematical operations on grdfiles in the frequency domain

**SYNOPSIS**

grdfftin_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**

grdfftwill 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-Moption that scales degrees to meters. If you have grdfiles with dimensions in km, you could change this to meters usinggrdeditor scale the output withgrdmath. No space between the option flag and the associated arguments. Use upper case for the option flags and lower case for modifiers.in_grdfile2-D binary grd file to be operated on.-GSpecify the name of the output grd file.

**OPTIONS**

-ATake the directional derivative in theazimuthdirection measured in degrees CW from north.-CUpward (forzlevel> 0) or downward (forzlevel< 0) continue the fieldzlevelmeters.-DDifferentiate 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, appendgto indicate that your data are geoid heights in meters and output should be gravity anomalies in mGal. [Default is no scale].-EEstimate power spectrum in the radial direction. Placexoryimmediately after-Eto 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. Appendwto write wavelength instead of frequency.-FFilter the data. Place x or y immediately after-Fto filter x or y direction only; default is isotropic. Specify four wavelengths in correct units (see-M) to design a bandpass filter; wavelengths greater thanlcor less thanhcwill be cut, wavelengths greater thanlpand less thanhpwill be passed, and wavelengths in between will be cosine-tapered. E.g.,-F1000000/250000/50000/10000-Mwill 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 (-) forhp/hcorlc/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.-IIntegrate 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, appendgto indicate that your data set is gravity anomalies in mGal and output should be geoid heights in meters. [Default is no scale].-LLeave trend alone. By default, a linear trend will be removed prior to the transform.-MMap 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 usinggrdproject.-NChoose or inquire about suitable grid dimensions for FFT.-Nfwill force the FFT to use the dimensions of the data.-NqwillinQuireaboutmoresuitabledimensions.-Nnx/nywilldoFFTonarraysizenx/ny(Mustbe>=grdfilesize).Defaultchoosesdimensions>=datawhichoptimizespeed,accuracyofFFT.IfFFTdimensions>grdfiledimensions,dataareextendedandtaperedtozero.-SMultiply each element byscalein the space domain (after the frequency domain operations). [Default is 1.0].-TCompute the isostatic compensation from the topography load (input grdfile) on an elastic plate of thicknesste. Also append densities for load, mantle, water, and infill in SI units. Ifte== 0 then the Airy response is returned.-Timplicitly sets-L.-VSelects 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)).

**SEE** **ALSO**

gmt(1gmt),grdedit(1gmt),grdmath(1gmt),grdproject(1gmt) 1 Jan 2004 GRDFFT(l)