Though the terms are often used interchangeably, bathymetry and hydrography are two distinct disciplines.
Bathymetry is the science of measuring and charting the depths of water bodies to determine the topography of a lake or river bed or of the seafloor. For centuries, mariners did this by relying on lead lines, compasses, sextants, and rudimentary nautical charts.
Bathymetry is the foundation of the science of hydrography, which studies the physical features of a water body, including its shoreline; the characteristics of tides, currents, and waves; and the physical and chemical properties of the water itself. Today’s hydrography includes the development of high-resolution digital terrain models of the seafloor, surveys that support seafloor construction, and data collection of a complete library of water column and oceanographic data that support models for a variety of uses.
More than 40% of the world’s population and 75% percent of all urban centers are in or near coastal environments. Therefore, population growth in coastal areas and sea level rise due to climate change are driving the need for bathymetric data for planning and emergency management. Additionally, more than 90 percent of the world’s trade is carried by sea. Therefore, accurate and up-to-date charts of coastal waters for ship navigation are vital for world commerce.
Yet, we know more about the surfaces of Mars and the Moon that we do about our ocean bottoms, only 10 percent of which we have properly surveyed and charted.
Bathymetric data collection, which experienced a boom with the advent of multibeam systems in the 1980s, is now experiencing a second renaissance due to the increase in maritime commerce, climate change, and advances in relevant technology. Most of the work in deep water is either research- and geology-based (such as by the oil industry for exploration) or for cable surveying. In shallower waters, habitat studies are a big reason. In even shallower waters, the issues are infrastructure—ports and harbors—and navigation. At depths of less than 30 meters, the largest drivers are navigation and dredging.
On land, surveyors face such challenges as rough terrain, human activity, and bad weather. Usually, however, the ground beneath their tripods doesn’t move and the air between them and their targets is transparent. Bathymetric surveyors, by contrast, work on vessels that are constantly rolling, pitching, yawing, heaving, swaying, and surging, due to waves, currents, and winds. Therefore, charting requires correcting for the vessel’s attitude, as well as the tidal datum and changes in the velocity of sound in the water (which depends on salinity).
Additionally, bathymetric surveyors are separated from the bottom by tens, hundreds, or thousands of feet of water, sometimes turbid and always impervious to GNSS signals. Therefore, underwater surveying relies very heavily on inertial data for positioning and navigation (except for underwater devices towed by a ship, which can use ultra short baseline systems to determine their underwater offset from the ship).
Yet another challenge is that ships that can carry large sonar systems cannot navigate in very shallow waters, while small vessels that can do so cannot carry that large equipment.
In deep waters, depth data is collected using huge multibeam echo sounders that operate at very low frequencies. As the depth decreases, smaller devices are used that operate at higher frequencies and, therefore, higher resolution. Very close to shore, where the shelf’s slope cuts off the cone of these devices’ sound signals, airborne green laser lidar sensors are a much more efficient means of collecting depth data. Multi-hull vessels are preferred for shallow-water bathymetric surveys, because they provide a stable platform and their shallower draft enables them to get closer to the shoreline.
Creating detailed and accurate models of underwater objects, such as sunken ships, requires a combination of multibeam scanning sonars, subsea lidar, and diver-portable sonar to create sonar and lidar point clouds.
Bathymetry from space consists of using satellite altimetry combined with high-resolution marine gravity information with available depth soundings from which scientists deduct the seabed morphology with a resolution from 1 km to 25 km. However, mapping ocean floors at the finest levels of detail still requires echo-sounding from ships.
The advent of unmanned aerial vehicles (UAVs) and autonomous underwater vehicles (AUVs), including gliders, has accelerated the effort to miniaturize bathymetric sensors.
Currently, we lack enough systematic surveys of the ocean with ships and autonomous vehicles to reach a level of detailed knowledge equivalent to what we have on land. Hopefully, the future of hydrography will include a much larger survey fleet, unmanned platforms, permanent seafloor installations, satellite remote sensing, and data collection by trusted sources, such as commercial vessels.
In fact, there is hardly an application better suited to crowd sourcing than bathymetry, considering that every commercial vessel is equipped with a very capable GPS receiver and an echosounder. Furthermore, commercial vessels have a vested interest in hydrographic data for safe navigation, can collect huge amounts of data during ocean passages, and are already transiting in the areas of interest, which significantly reduces acquisition costs.