Remote sensing using laser projection photogrammetry for underwater surveys

Washington Department of Fish and Wildlife researchers combined the Laser/Video Photogrammetric System (originally presented at MTS/IEEE OCEANS 2002 Conference) with precision navigation to provide an efficient "tool" for producing quantitative benthic survey data. In addition to the roll/pitch motion reference sensor, CCD video camera and three microlasers utilized in the original system, a Doppler velocity log (DVL), ring laser gyro (RLG), ultra short base line sonar (USBL) and Integrated Positioning System (IPS) software were added to provide more precise navigation. This precision allows for geo-referencing, and hence mosaicking of user specified sized areas within an image along a transect survey. Two additional lasers were also added to test for improvement of scale measurements. Custom software is used to automatically process video data at user selected distance intervals. The software locates the reference lasers in an image and optical triangulation is used to compute range to a bottom plane in the field-of-view. Using this information and measurements recorded from the motion sensor, the spatially variant magnification is determined over the entire field-of-view using a simple algorithm. As a result, a variety of parameters are estimated using image-processing techniques including: perspective overlays, range to a point or location, scale in any region of the image, and area measurements. This paper briefly reviews the original system, describes the enhancements, and summarizes data from two cruises. In the most recent cruise during October, 2003 the system was deployed in the Channel Islands National Marine Sanctuary located off the coast of southern California. Mounted onboard the DSV DELTA submersible, the system collected numerous hours of video and navigation data from survey transects. Data were collected from both randomly sampled sites and selected areas where ground-truth targets were placed. Methods to improve these results are also discussed. Deployment of a third generation system is expected in 2005 onboard an AUV. We expect the final system will prove valuable for estimating the abundance of commercially and recreationally exploited groundfish species within a study area using stratified random video transect methods. This unintrusive, direct observation technique affords a means to estimate the density of certain benthic fish species in high relief areas that are not accessible to routine trawl survey methods. Differences in fish densities between trawlable and untrawlable habitats have clear implications for estimation of abundance for a number of fish species including rockfish.