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Background theory on bathymetric Lidar can be found in the paper, "Meeting the Accuracy Challenge in Airborne Lidar Bathymetry" (Guenther et al.). However, in general, the laser outputs a green and infrared beam. The infrared beam is used to detect the water surface and does not penetrate this. The green beam penetrates the water and is used to detect the seafloor. The green beam also generates red energy when excited at the air / water interface. This is known as Raman backscatter and can also be used to detect the sea surface. Distances to the sea surface and seafloor are calculated from the times of the laser pulses, using the speed of light in air and water. The 1kHz bathymetric laser (or hydro laser) is used to collect bathymetric sounding data over water. The system meets IHO Order 1 survey accuracy specifications and can be configured to operate at four different sounding densities: 2m x 2m, 3m x 3m, 4m x 4m and 5m x 5m. There is a very close relationship between sounding density and cost. In general terms, a greater sounding density results in a lower production rate, which leads to a more expensive survey. View table showing the relationship between sounding density, swath width and production rates. The 10kHz topographic laser is used to collect elevations over the exposed land and along the shoreline of the survey area inshore to width required. The system collects topographic elevations to an accuracy of +/- 25cm (1 s) and can be configured to operate at many different spot densities ranging from 2m x 0.7m through 2m x 3.2m. Again, there is a close relationship between spot density and cost, though it is nowhere as dramatic as that for the bathymetric collection. |
Data received by the airborne system are continually monitored for data quality during acquisition operations. Display windows show coverage and information about the system status. In addition, center waveforms at 5Hz are shown. All of this information allows the airborne operator to assess the quality of data being collected.
In addition to Lidar data, a DuncanTech DT4000 digital camera will also be used to acquire one 24-bit color photo per second. The camera, mounted in a bracket at the rear of the sensor, captures imagery of the area being over flown, and can be used during post-processing.
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