grdfft - Perform mathematical operations on grdfiles in the frequency domain grdfft in _ grdfile -G out _ grdfile [ -A azimuth ] [ -C zlevel ] [ -D [ scale |g ] ] [ -E [ x|y ][ w ] ] [ -F [ x | ... The horizontal dimensions of the grdfiles are assumed to be in meters. ... 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]. ...