Ground-Truth Results of Comparison Airborne Lidar Bathymetry (ALB) in Sub Tidal Coastal Environments

Abstract

Results from a recent study of the performance of ALB in the coastal waters of New Hampshire-Maine indicate a potential relationship between lidar-laser measurements and seafloor characteristics. Comparison of bottom-detection results from two different ALB systems (Tenix LADS and Optech SHOALS) in the Portsmouth Harbor, NH and offshore Gerrish Island, ME showed a striking correlation in the lack of bottom detection in shallow waters (3-25 m). This lack of bottom detection is independent of the tide state, the date of data collection and the direction of the survey flight. Multibeam-echosounder measurements (Simrad EM3002) were used as reference measurements. The multibeam results show that the commonly used assumption is incorrect that where no bottom detection was made by the lidar system, the bottom depths are deeper than their surroundings. Bathymetric and acoustic-backscatter maps from the multibeam echosounder shows that the areas of no bottom detection by lidar are shallow and are affected by the seafloor characteristic. Two potential conceptual models were developed to explain the lack of bottom detection: one is dominated by the seafloor grain size and the second one is dominated by the presence of vegetation. Underwater video imagery and bottom samples were taken from the lack-of-detection areas and their surroundings in the summer of 2008. The underwater imagery shows that in some locations aquatic vegetation is present, but its distribution was very sparse. Also, the underwater imagery shows that areas where successful ALB-bottom detection occurred, the bottom is sandy, whereas the seafloor that showed lack of detection is composed of gravel, pebbles and outcrops. Preliminary analysis characterized the sands near Gerrish Island as a grayish-olive medium- to very fine-grained sand (2 to 3.5 phi) and the sands in Portsmouth Harbor as dark yellowish-brown very fine pebble gravel to coarse sand (-1 to 1.5 phi). The areas that show a lack of detection vary in color and grain size, where the common characteristic is that all areas have pebbles, gravel or outcrops (larger than-1 phi). Preliminary conclusions show that the dominant seafloor characteristic affecting the lidar bottom detection in shallow waters is the grain size of the seafloor sediments. The color of the seafloor does not appear to affect the lidar performance as much as grain size.

Department

Center for Coastal and Ocean Mapping

Publication Date

6-2009

Journal Title

GeoTech

Conference Date

Oct 5 - Oct 6, 2009

Publisher Place

Silver Springs, MD, USA

Publisher

ASPRS

Document Type

Conference Proceeding

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