USGS Scientists Develop System for Simultaneous Measurements of Topography and Bathymetry in Coastal Environments
Researchers with the U.S. Geological Survey (USGS) Alabama Water Science Center and the USGS St. Petersburg Coastal and Marine Science Center met in Biloxi, Mississippi, July 11-15, 2011, to assemble and test a boat-mounted system that simultaneously measures topography (onshore elevations) and bathymetry (seafloor depths) in nearshore environments. The USGS Ecosystems Program funded the project.
The components of the system are a light-detection-and-ranging (lidar) instrument for measuring onshore elevations and an interferometric sonar for measuring seafloor depths. The lidar, an Optech ILRIS HD-ER-MC, is fixed to the top of a shallow-draft vessel to scan the shoreline with a laser beam in a horizontal direction. This instrument is capable of producing three-dimensional point-cloud images of the terrestrial environment, as demonstrated in a recent analysis of historic live oaks in Auburn, Alabama (see http://al.water.usgs.gov/tlidar/toomersoak.html). The interferometric sonar, a SEASwath 468H, provides not only high-resolution seafloor bathymetry (depths) but also backscatter (a proxy for seafloor texture; for example, see Sound Waves story "Storm Impact, Sea-Floor Change, and Barrier-Island Evolution: Scientists Map the Sea Floor and Stratigraphy Around Ship and Horn Islands, Northern Gulf of Mexico"). An onboard Global Positioning System (GPS) receiver (an Ashtech differential GPS system) uses satellite signals to determine the vessel's position, and a motion sensor (Applanix POS MV L1/L2) provides highly accurate data about the vessel's attitude, heading, heave, position, and velocity.
Both the lidar and the sonar operate on similar principles: each instrument emits energy—light from the lidar and sound from the sonar—that reflects off a point, or "target," and returns to a receiver on the instrument. Knowing the direction the energy travels, its speed, and the time it takes to travel to the target and back allows calculation of the target's position relative to the instrument. To get the desired information—the target's position in three-dimensional space (for example, its elevation, latitude, and longitude)—the instrument's position in three-dimensional space must be determined. Furthermore, to integrate the topographic and bathymetric datasets, the relative positions of the lidar, sonar, and motion sensor also must be precisely determined. To facilitate these determinations, the instruments were mounted in a vertical line, and their positions relative to each other and to the vessel's GPS receiver were measured by using a "total station"—a tripod-mounted assembly of electronic instruments used in surveying.
During a marine survey, determination of the position of the vessel must accommodate motion in any direction. The GPS tracks vertical and horizontal movement, while the motion sensor keeps track of pitch and roll; and the inline position of the instruments facilitates instantaneous calculations because it reduces the angular movement of each instrument relative to the others. The fixed-position information from the total-station data and the motion data from the onboard sensors allow calculation of the lidar's and sonar's positions while mapping is underway. Through calculating the position of the vessel and the calibrated speed and trajectory of light and sound, the positions of targets on land and in the water can be determined simultaneously.
Before the survey, the lidar was calibrated by measuring static (unmoving) targets of known distance, again using a total station. During the survey, the sonar was calibrated to the acoustic properties of the water by repeatedly measuring sound velocity, which is affected by changes in salinity, turbidity, and temperature. Finally, an Ashtech GPS base station deployed over a nearby bench mark provided data to correct for atmospheric distortion of the GPS signals. The data streams from the lidar and the sonar were acquired and processed by using Applanix POSPac MMS and Hypack HySweep software.
On hand for the experiment were Athena Clark, Dustin Kimbrow, and Kathyrn Lee from the Alabama Water Science Center, and Dana Wiese, BJ Reynolds, Kyle Kelso, and Jim Flocks from the St. Petersburg Coastal and Marine Science Center. Joe Revelle and Angie Pelkie from Optech and Harold Orlinsky from Hypack assisted the scientists.
To test the system, the team conducted a survey along Deer Island, a small barrier island immediately offshore of Biloxi. The island recently underwent restoration by the U.S. Army Corps of Engineers, and the test run can be used to develop baseline elevation data for this restoration effort. Once they were certain the system was operational, the team traveled to the remote Chandeleur Islands to survey a recently constructed oil-spill-mitigation sand berm. The initial intent of this 100-m-wide, 26-km-long manmade feature was to trap oil from the 2010 Deepwater Horizon oil spill. Since then, the berm has become of significant interest to coastal scientists and managers as a proxy for barrier-island response to storm impacts, and a useful site for observing and modeling the potential contributions of manmade structures to fragile barrier-island systems. The St. Petersburg Coastal and Marine Science Center has conducted extensive airborne lidar and bathymetric surveys around the sand berm over the past year; data from these surveys can be compared with data from July's integrated lidar-sonar mapping to assess the accuracy of the mapping.
The lidar and sonar systems provided high-resolution imaging of the subaerial and submerged extent of the islands. The experiment demonstrated that the systems can be integrated and rapidly deployed in shallow-water environments to obtain extremely accurate elevation measurements for modeling efforts and studies of coastal change over time. The collaboration and mutual enthusiasm of team members provided an enjoyable and educational field experience, and several research opportunities are being explored that can benefit from this integrated "topobathy" system.
in this issue:
New System Measures Topography and Bathymetry Simultaneously