Bathymetric LiDAR Point Densities and the Point of Diminishing Returns
In bathymetric lidar, higher point density does not always equate to greater seafloor detail in the final point cloud. Bathymetric lidar beam divergence determines the size of the laser footprint on the sea surface. Water clarity and depth contribute to the spreading of the laser pulse through the water column, so that the size of the laser footprint on the seafloor can be as great as 1/2 the water depth. Receiver telescope field of view controls the area on the water surface from which photons are recorded. Water clarity and depth “propagate” the field-of-view so that it spreads in area with depth as well. Theory can predict the depth at which adjacent laser pulses are essentially measuring depth over the same area of the seafloor. For example, it is predicted that adjacent measurements of the shallow channel (1.7 milliradians) in the Coastal Zone Mapping and Imaging Lidar (CZMIL) are overlapping by about 80% at 10 m water depth in clear water. These types of predictions can help us understand when higher density systems can actually produce higher resolution data, as opposed to just more points containing the same information. The US Army Corps of Engineers and the National Oceanic and Atmospheric Administration have collected or contracted bathymetric lidar datasets using systems with differing beam divergence and fields-of-view (CZMIL, VQ820-G, VQ-880-G, HawkEye III) in similar environments. These datasets give us a unique opportunity to validate our theoretical work in a couple of environments. The coast of Puerto Rico has hard bottom structure ranging from shallow rocky shoreline to coral reefs, and a few shipwrecks. The Gulf Coast of FL is relatively homogeneous sandy area with some seagrass. This presentation will detail analyses of these datasets to compute point density, and how that relates to the ability to resolve the shape and size of submarine features of various dimensions and depths. Results of this study can inform specifications for bathymetric and topo-bathymetric lidar data acquisition and for design of new systems. It can also be used to identify a depth cut-off for reporting depths from higher-resolution shallow-water systems when they are flown in tandem with lower resolution deep water systems, or in hybrid systems that contain both capabilities.