In the digital analysis of SEASAT synthetic aperture radar (SAR) imagery of ocean waves, it is desirable to first correct sensor-induced geometric distortions. Chief among these is the nonlinear range distortion due to the presentation of optically-processed SAR imagery in the slant range plane. For the average SEASAT case of 200 incident illumination angle, range distances are severely compressed, leading to inordinately short wave periods in the imagery's range direction. An efficient digital filtering algorithm that accurately corrects for this distortion has been developed; from a set of equally-spaced slant plane digitized samples the algorithm produces a set of equally-spaced ground plane samples using a table look-up filtering procedure. Consequently, customary FFTs, which operate on uniformly sampled data, may be employed in detecting dominant wave periods and directions from the geometrically corrected SAR imagery. Further, by converting imagery collected at differing illumination angles to a common coordinate system, the correction makes possible a useful comparison of spectra obtained from nonoverlapping image regions, thereby increasing the reliability of the detection decision.