Lidar Survey

The Bureau acquired airborne light detection and ranging (lidar) data along two transects across Mustang Island, Texas. One transect crosses the southwest part of the island at Mustang Island State Park. The other transect is located southwest of Port Aransas. The purpose of this mapping was to evaluate the usefulness of lidar to delineate extents of coastal habitats based on subtle topographic changes. Data sets include digital elevation models (DEMs) created from lidar point data. The DEMs lie within five U. S. Geological Survey 7-1/2 minute quadrangles between Port Aransas on the northern end of Mustang Island and Padre Island National Seashore on the southwest.

Lidar digital elevation points are computed using three sets of data: laser ranges and their associated scan angles, platform position and orientation information, and calibration data and mounting parameters (Wehr and Lohr, 1999). Global Positioning System (GPS) receivers in the aircraft and on the ground provide platform positioning data. The GPS receivers record pseudo-range and phase information for post-processing. Platform orientation information comes from an Inertial Measurement Unit (IMU) containing sets of three orthogonal accelerometers and gyroscopes. An aided-Inertial Navigation System (INS) solution for the aircraft's attitude is estimated from the IMU output and the GPS information.

The DEMs were derived from lidar x, y, and z point data generated by combining laser range and aircraft attitude data collected using an Optech Inc. Airborne Laser Terrain Mapper (ALTM) 1225 with once per second data collected using geodetic quality GPS airborne and ground-based receivers. The Bureau's ALTM 1225 system was installed in a single engine Cessna 206 aircraft owned and operated by the Texas State Aircraft Pooling Board, and operated from the Aransas County Airport in Fulton, Texas. The lidar data were collected during two flights on September 18 and October 31, 2003. Lidar instrument settings and flight parameters were: (1) laser pulse rate 25 kHz, (2) scanner rate 35 Hz, (3) scan angle +/-15 degrees, (4) flight altitude 450 to 665 m, and (5) ground speed 90 to 110 kt. At least two GPS base stations were operated during each flight.

We produced DEM swaths several hundred meters wide from the gulf beach to the bay shore for the Mustang Island State Park and Port Aransas transects. Horizontal coordinates are in the Universal Transverse Mercator projection using the 1983 North American Datum, zone 14, 1980 Geodetic Reference System. Elevations are relative to the 1988 North American Vertical Datum (NAVD). Lidar-derived elevations have a horizontal and vertical accuracies estimated at 0.01 to 0.03 m from comparisons with ground GPS surveys. Horizontal agreement between the ground kinematic GPS and the lidar was within the resolution of the 1 m x 1 m DEM.

The lidar data were sorted to extract points within 0.5 m of a ground GPS survey point. The mean difference between elevations derived from lidar and ground GPS was used to estimate and remove an elevation bias from the lidar. The standard deviation of these elevation differences provides an estimate of the lidar precision of 0.13 ± 0.04 m. After removing vertical biases from each flight, a vertical uncertainty of 0.04 m (root mean square) remains.

We produced detailed elevation profiles along each transect for comparison with wetland maps, vegetation surveys, and conductivity measurements, and generated averaged elevations for all lidar data points within 1.5 m of a transect station where we also measured ground conductivity and vegetation abundance, type, and height.