A coupled topographic/bathymetric surveying system has been developed which integrates a terrestrial scanning laser with acoustic sonar to provide a complete picture of coastal and riverine structures and landscapes. A boat is used as the survey platform. Integration was performed in support of the Conditioning Index Work Unit, Navigation Systems General Investigations R&D Program, which is developing standards and tools for assessment of coastal structures. The purpose of this work unit is to provide Corps of Engineer Districts with a toolbox to inspect, evaluate, and manage critical navigation assets.
A Riegl LMS-Z390i and the V400 Terrestrial LIDAR was integrated with a 250 and 500 KHz GeoSwath Plus Interferometric Multi-Beam Sonar, and an Applanix PosMV inertial motion unit (IMU). Real-time GPS corrections were supplied by a Trimble R8 GNSS and broadcast to the PosMV for positioning and crucial time tagging of all instruments. The Geoswath data were collected using its proprietary software package and the Lidar data were collected using Hypack hydrographic software. The Geoswath and Riegl sensors receive real-time corrections from the PosMV unit; therefore resulting in properly georeferenced coordinates for both the above- and below-water data sets.
This project demonstrated that the lidar could be relatively easily integrated into the existing IMU/sonar system. Data acquired by both sensors was successfully organized into a point cloud representing the coastal landscape including infrastructure. A work flow employing software products was defined to organize the point cloud based on real-time trajectory data. The resulting organized point cloud was of extremely high quality with respect to the accuracies of the system components. Further post processing of trajectory data would improve data quality.
The integrated system will allow the Corps to perform asset management functions quicker and for much less money. GeoSwath Plus allows wide swath coverage in shallow water, up to 12 times water depth; integrated bathymetry and simultaneous side scan; and high depth resolution. The manufacturer stated resolution of the GeoSwath Plus is 3 mm at 50 meters. The Reigl Terrestrial Lidar has a ranging accuracy rated at 2 mm at a distance of 400 meters. In the demonstration process position data collected with the R8 GPS were compared to position data collected with the integrated survey system; results matched within the accuracy of the laser. The major source of error to this integrated approach is the accuracy of the RTK GPS.
The capability that is being developed includes tools and protocol for surveying of many different types of Corps hard structures; it is applicable to soft structures as well, such as beaches, dunes, levees. Any change to a structure will be detected by performing an initial survey and comparing results with those from a follow-up return survey. Damage and/or routine scour at structures will be detectable at very high resolution, more quickly, and at much lower cost. Above- and below-water surveys can be conducted concurrently. High resolution point-clouds can be analyzed using many standard commercial software products.
Another potential spin-off product from the integrated surveying technique is development of scene data for ship simulator studies when investigating navigation safety issues, especially in the areas of accident reconstruction. These simulator studies need close-to-reality scenes so the operators can make adjustments to the operation of the vessel as they navigate through the water way. Some possible military applications would be in the identification of underwater submerged obstructions and the detection of surface anomalies along the side of a road or along a waterway, such as IEDs.
U.S. Army Corps of Engineers Research and Development Center, Coastal Hydraulic Laboratory, Vicksburg, MS. Thad Pratt, 601-634-2959, Thad.C.Pratt@usace.army.mil, William C. Butler, 601-634-2345, William.C.Butler@usace.army.mil