Underwater base maps—bathymetric charts—are made by echosounding. Early echosounding was done using a single sonar beam pointed at nadir below a survey vessel. Today, the recognized standard for acquiring bathymetric data is a multibeam echosounder (MBES)—see "Sonar in Action," above.

Navigation safety is a primary echosounding application, and the technology often is used to make nautical charts for the National Oceanic and Atmospheric Administration. Such charts can take the form of underwater terrain maps or benthic habitat maps. The U.S. Army Corps of Engineers uses echosounding to construct dredged channels for safe navigation. Marine scientists and coastal resource managers use bathymetric charts and backscatter imagery as a basis for their work,  as illustrated in "MBES Mapping Details Fertile Fishing Grounds, " below.

Mapping Underwater with Light
Light, in the form of underwater photography from a submersible, contributed to the groundtruthing process for the Fairweather Ground benthic habitat mapping. Sunlight provides a basis for coastal zone benthic habitat mapping, as detailed in "Shedding New Light on Underwater Habitats," Earth Imaging Journal, July/August 2008. In that case, underwater habitat classification was accomplished by a team of marine researchers who used digital airborne imagery and applied semi-automated image classification technology to map seagrass beds along the Texas coast.

The green laser light of a hydrographic LiDAR offers a powerful underwater mapping tool wherever water conditions are favorable. LiDAR can deliver hydrographic-quality data for charting, and the technology is used for coastal zone bathymetry mapping for environmental and coastal protection applications. For this discussion, hydrographic LiDAR is referred to as "airborne LiDAR bathymetry (ALB)."

Whenever possible, ALB surveying is best done before MBES data acquisition. Obtaining shallow and sometimes navigationally dangerous areas will result in a more efficient underwater mapping program. As shown above, MBES surveying becomes less efficient as the depth shoals, as the data swath narrows with less depth. Moreover, reefs and shoals can be a threat to vessel operations. By surveying as much as possible by ALB first, the subsequent MBES surveying can be planned to provide the overlap needed with the hydrographic LiDAR data set. 

ALB augments the acoustic data set nearshore by picking up the depths and extending them across shallow, dangerous zones and onto the land. This technology is especially effective in tropical waters with coral reefs, as well as areas where there are complex coastlines and shoals to map. ALB also yields laser reflectance imagery from the submerged terrain, which can be better mapped for geological and habitat applications.

As discussed in "ALB Hastens Coastal Surveys," above,  ALB can be uniquely useful for surveying dams or coastal structures, which are commonly "amphibious." The integrated green and infrared lasers provide complete and continuous elevations from the submerged base to the subaerial top. In addition, ALB can be used to evaluate river conditions, as detailed in "Monitoring Rivers with LiDAR," above.

Combining acoustic and LiDAR surveying represents a safe and efficient way to perform underwater mapping applications as well as resource management and engineering underwater surveys., especially in areas of complex shorelines and shoals nearshore.  As detailed in "Creating Tools for Safer Navigation" above, combining multibeam and hydrographic LiDAR operations is the future in hydrographic surveying for navigation safety mapping applications, as well as resource management and engineering underwater surveys.

Ongoing Development
Underwater mapping will continue to evolve to meet environmental and engineering challenges. For example, the deep waters now being explored for oil require the high resolution obtained easily only in shallow water. As shown in the images at the beginning of this article, new survey systems have been launched on autonomous underwater vehicles to bring high-resolution seabed survey results from depths to 3,000 meters.


Underwater mapping technology will continue to probe deeper and farther with ever-increasing resolution levels.

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