During the SEASAT repeat-orbit period, a CTD and deep (1800 m) XBT hydrographic experiment was conducted across the Kuroshio to obtain cotemporaneous surface topography measurements for comparison with the altimeter-derived surface height measurements. The results showed that the altimeter could detect and measure the change in sea surface topography caused by movement of the Kuroshio and the cold rings that were found in the hydrographic data. After compensation for the small diffrences in the time of the measurements, the altimeter measured changes in the surface topography that matched those measured by the hydrographic sections with an average difference of 9.6 cm for the 41 comparison points.