Walter H.F. Smith NOAA Laboratory for Satellite Altimetry • Silver Spring, Maryland USA David T. Sandwell Institute for Geophysics and Planetary Physics, Scripps Institution of Oceanography University of California at San Diego • California USA Oceanography • Vol. 17 • No. 1/2004 8 Special Issue—Bathymetry from Space Conventional Bathymetry, Bathymetry from Space, and Geodetic Altimetry become highly refined since the 1980s and now systems can map a swath of area beneath a ship’s track with a width as much as twice the water depth in deep water. However, the speed of the ship is limited, and thus also the rate at which ocean area may be mapped. A com- plete swath survey of the deep ocean would take about 200 years of survey time, at a cost of billions of dollars (Carron et al., 2001); shallow coastal areas would take even longer. Estimates of how much ocean floor is already mapped by swath bathymetry vary because some data are classified military secrets or proprietarily held by their collectors. Publicly available data cover only a few percent of the ocean floor, and there is general agree- ment that even if all data became public, they would still cover only a small fraction of the deep ocean area. If a complete global survey could be made by swath mapping, it would have much higher resolution and accuracy than what can be done from space. Until such a survey is a reality, however, we must work with the available data, which are primarily older, “low-tech” analog echosoundings. Historically, the mandate for soundings has come from the need to chart hazards to navigation, that is, bottom features that are so shallow that a ship could run aground on them at low tide. This naturally con- centrates mapping efforts very close to shorelines. More recently, there has also been some interest in map- ping exclusive economic zones (EEZs), which extend outward 200 nautical miles from shore. The distribu- tion of soundings in the ocean is relatively dense in shallow coastal areas and EEZs, but very sparse in the open ocean. As Figure 2 shows, the distribution of sur- vey lines covers the South Pacific as coarsely as the Interstate Highway System covers the United States. Echosounding data also have an uneven geo- graphical distribution of technology and quality. Most of the soundings in remote oceans are old analog measurements geo-located using only celestial naviga- tion (Smith, 1993). Modern digital swath systems with This article offers a general introduction to those aspects of bathymetric mapping and satellite altimetry that are relevant to bathymetry from space. We begin with a review of some of the strengths and weaknesses of conventional bathymetric measurement and map- ping. This context highlights the case for and value of space-based mapping: it is the only way to achieve globally uniform resolution within reasonable time and cost. However, a space mission cannot “see” the ocean floor directly; instead, it observes gravity anom- alies that can be correlated with ocean floor topogra- phy. Geological factors and physical laws limit the res- olution of this technique to a particular range of spatial scales (~100 km to ~5 km). While this is not perfect, it yields an enormous improvement in the resolution of global bottom roughness over traditional methods (Figure 1). A satellite altimeter mission designed for bathy- metric mapping is simpler and cheaper than one designed to monitor ocean currents, tides, or climate. It also yields information about Earth’s gravity field that is independently useful for resource exploration and for compensation of the errors in inertial navigation systems. A new mission with a state-of-the-art altime- ter could optimize the mapping of gravity and bathymetry and resolve a key element of bottom roughness—abyssal hill orientation—for only $100M. More complete and technical reviews of these top- ics may be found elsewhere. Smith (1993) reviewed the problems and errors in conventional bathymetric data. Details on the processing of altimeter data to yield gravity and bathymetry may be found in Smith and Sandwell (1994; 1997), Sandwell and Smith (1997; 2001), and Smith (1998). Chelton et al. (2001) present a thorough treatment of satellite altimetry, with a view toward measuring ocean currents and climatic signals. Conventional Bathymetric Measurements Direct measurement of ocean floor depth is done by echosounding from a ship. This technique has This article has been published in Oceanography, Volume 17, Number 1, a quarterly journal of The Oceanography Society. Copyright 2003 by The Oceanography Society. All rights reserved.Reproduction of any portion of this article by photocopy machine, reposting, or other means without prior authorization of The Oceanography Society is strictly prohibited. Send all correspondence to: info@tos.org, or 5912 LeMay Road, Rockville, MD 20851-2326, USA.