Bathymetry

Bathymetry is the science of measuring and mapping the depth of water bodies, including the floors of oceans, rivers, and lakes. It is the underwater counterpart to hypsometry (the study of land elevation) and topography. Evidence of depth measurements dates back over 3,000 years to Ancient Egypt. Applications of bathymetry include creating charts for safe navigation and identifying underwater hazards, studying marine life near the seabed, analyzing coastlines, and understanding ocean dynamics such as currents and tides.

Bathymetric charts, distinct from hydrographic charts, are designed primarily for surface or subsurface navigation safety. They typically depict seafloor relief using contour lines (known as depth contours or isobaths) and selected depth measurements called soundings, often alongside surface navigational information. Bathymetric maps, a broader term used when navigation safety is not the focus, may employ digital terrain models and artificial lighting to represent depths. Global bathymetry is sometimes combined with topographic data to create a global relief model. Paleobathymetry is the study of historical underwater depths.

Synonyms for bathymetry include seafloor mapping, seabed mapping, seafloor imaging, and seabed imaging. Measurement methods range from depth sounding, sonar, and lidar to buoys and satellite altimetry. The choice of method depends on the area’s scale, budget, desired accuracy, and other factors. Despite modern technology, large portions of the ocean floor remain less mapped than the surface of Mars.

Originally, bathymetry relied on depth sounding using a pre-measured heavy rope or cable lowered from a ship. This method measures depth one point at a time, making it less efficient and less accurate due to ship movement and currents.

Today, most bathymetric data comes from echosounders (sonar) mounted on boats, which send sound beams downward, or from airborne lidar or ladar systems. The time for sound or light to travel to the seafloor and back determines the distance. Single-beam sounders were used from the early 1930s, but modern multibeam echosounders (MBES) use hundreds of narrow adjacent beams (often 256) arranged in a fan-like swath of 90 to 170 degrees. This provides high angular resolution and accuracy. A wide swath allows a boat to map more area in fewer passes than a single-beam system. Beams update many times per second (typically 0.1 to 50 Hz, depending on depth), enabling faster boat speeds while maintaining full coverage. Attitude sensors correct for the boat’s roll and pitch, and a gyrocompass corrects for yaw; most modern systems integrate motion sensors and positioning. A satellite navigation system positions soundings relative to Earth’s surface. Sound speed profiles correct for refraction caused by variations in water temperature, conductivity, and pressure. A computer processes all data, correcting for these factors and each beam’s angle. Resulting soundings are processed manually, semi-automatically, or automatically to produce maps. As of 2010, outputs include selected soundings or digital terrain models (DTMs), which are grids of points forming a surface. Historically, soundings were more common in hydrography, while DTMs were used for engineering, geology, and flow modeling. Since around 2003–2005, DTMs have become more accepted in hydrographic practice.

Satellites also measure bathymetry by detecting subtle sea-level variations caused by the gravitational pull of underwater features like mountains and ridges. Sea level is generally higher over such features than over abyssal plains and trenches.

In the United States, the Army Corps of Engineers oversees most surveys of navigable inland waterways, while NOAA handles ocean waterways. Coastal bathymetry data is available from NOAA’s National Centers for Environmental Information. Bathymetric data is usually referenced to tidal vertical datums. Deep-water data often uses Mean Sea Level (MSL), but nautical charting data in the U.S. typically uses Mean Lower Low Water (MLLW), while other countries use Lowest Astronomical Tide (LAT). Many other datums are also used in practice.