Mapping the ocean floor

An Earth-orbiting radar cannot see the ocean floor, but it can measure ocean-surface height variations induced by the topography of the ocean floor. The gravitational pull of the seafloor produces minor variations in ocean surface height. Seafloor mapping by ships is much more accurate than radar altimeter mapping, but to date only 10% of the seafloor has been charted this way. A complete mapping of the deep oceans using ships would take 200 ships navigating Earth, 24 hours a day, for an entire year at a cost of billions of dollars. Mapping using satellite radars can cover a larger area in a shorter amount of time. When interesting features are discovered in satellite measurements, they can later be surveyed in fine detail by ships.