Subtask 4.2: Definition of High Resolution (Lidar) Northern Gulf Coast Geomorphology

Subtask Leaders: C. Wayne Wright - USGS Florida Integrated Science Center, St. Petersburg
Amar Nayegandhi - Jacobs Technology, Inc. /USGS Florida Integrated Science Center, St. Petersburg

Subtask 4.2 Aim

Lidar Data Collection: The NASA Experimental Advanced Airborne Research Lidar (EAARL) collects georectified digital aerial photographs and high-resolution lidar data. The instrument uses a green laser and a raster scanning mechanism to acquire lidar data. Researchers set up a GPS base station to precisely locate the position of the aircraft and the data it collected. [enlargement]

Following the devastating hurricanes of 2005, the geomorphological structure of the Northern Gulf of Mexico (NGOM) landscape has undergone significant change. Detailed mapping of present day geomorphology of the NGOM region can provide quantitative indicators to assess risk-prone lands and baseline data for future landscapes and ecological changes. The geomorphic significance of catastrophic events to coastal geomorphology can be quantified by determining fine-scale topography using airborne laser altimetry or Light Detection and Ranging (Lidar) techniques.

The NASA Experimental Advanced Airborne Research Lidar (EAARL) instrument provides unprecedented capabilities to survey near-shore benthic habitats, coastal vegetation and sandy beaches. Radiating in the blue-green portion of the electromagnetic spectrum, the EAARL is specifically designed to measure near-shore shallow submerged topography and adjacent coastal land elevations in a single scan of transmitted laser pulses. Post-flight processing of EAARL data is accomplished using a custom-built NASA-USGS Airborne Lidar Processing System (ALPS) that combines laser range measurements digitized at 1-nanosecond intervals with aircraft positioning data derived from an inertial measurement unit and precision Global Positioning System receivers. The ALPS software provides unique capabilities to explore and process Lidar return waveforms, and enables the creation of Digital Elevation Models (DEMs) for bare-Earth, canopy-top, and submerged topography. The EAARL system also incorporates a color infrared (CIR) downlooking digital mapping camera that continuously acquires digital aerial photographs during Lidar surveys.

EAARL surveys have been conducted post- Hurricane Katrina over Chandeleur Islands, the barrier islands of Gulf Islands National Seashore, and Jean Lafitte National Park. Additional Lidar surveys have been conducted in the NGOM study region by several federal, state, and local agencies, using other government and commercial Lidar sensors.  These surveys resulted in data sets processed in different datums, spatial resolutions, and output data formats. The evaluation and synthesis of these previously collected data sets are necessary to understand gaps in data coverage and any limitations in the existing data.

Traditional discrete-multiple-return, small-footprint Lidar systems are unable to detect small vertical variations due to the significant ‘dead zone’ between successive reflections from a transmitted laser pulse. The dead zone is usually caused by hardware limitations in the sensor design.  Results from determining bare-Earth topography under needle grass using the EAARL system at the Terra Ceia preserve in Tampa Bay, Florida suggests that waveform-digitization of the reflected laser pulse followed by range determination in post-flight processing software is a possible method to distinguish between ground elevations and canopy- top elevations of short vegetation in coastal wetlands.  

Given the unique requirements of the NGOM project and other CMG programs in obtaining accurate, high-resolution submerged and sub-aerial topography in near-shore coastal environments, and the lack of existing commercial sensors that provide these capabilities, the primary focus of this subtask is to establish lidar capabilities within the USGS CMG program. The EAARL sensor and processing software will be made available to USGS NGOM and other CMG projects. The current design of the EAARL sensor allows water-penetration to slightly more than one Secchi disk depth. As part of Subtask 4.2, hardware and software improvements will be made to the EAARL system to improve the water- penetration capability, thereby enabling improved simultaneous mapping of hydrologic and topographic features in the Northern Gulf of Mexico. The ability of the EAARL system to map bare-Earth topography under a variety of vegetation communities, including short shrubs and grasses, makes it an ideal mapping tool for storm impact studies and flood inundation predictions in wetlands. These unique capabilities of the EAARL sensor require additional software enhancements to glean more information from the waveforms that describe the vertical structure of vegetation canopies.

Subtask 4.2 Activities:

The objectives of this sub-task are to:

• Evaluate and synthesize existing topographic and near-shore bathymetric data acquired within the NGOM project study area post Hurricane Katrina in collaboration with the USGS Earth Resources Observation Systems Data Center.
• Establish lidar capabilities within the USGS CMG program to enable USGS EAARL surveys for the NGOM project and other CMG projects.
• Advance shallow water submerged topographic mapping capability of the EAARL system in order to map the hydrologic and topographic features at desired sites within the NGOM region.
• Define site locations to conduct high-resolution EAARL lidar topographic surveys based on requirements and feasibility.
• Acquire and process any additional lidar data and associated imagery acquired by the EAARL system in support of the NGOM project.

Subtask 4.2 Methodology:

In a close collaboration with the EROS Topographic Sciences team, all post-Katrina and post-Rita lidar data will be assembled into a seamless 1/9-arc-second (3-meter) elevation dataset. This topographic dataset will include lidar data collected by NASA, NOAA, USACE, and other state and local groups in the NGOM region. These data, along with the existing 1/9-arc-second National Elevation Dataset layer developed from pre-Katrina lidar collections, will be useful for characterizing vegetative, hydrologic, and geomorphic changes to the NGOM landscape resulting from the hurricanes. EROS topographic sciences staff will use topographic analysis tools to map, describe, and quantify these changes.

Conduct Lidar Surveys:

Another focus of this sub-task is to establish EAARL lidar capabilities within the USGS CMG program in support of NGOM and other CMG projects. A new aviation capability will be established in accordance with DOI National Business Center Aviation Management Directorate, headquartered in Boise, Idaho. Contracts will be established for aviation services for the experimental lidar, photographic, hyperspectral and other remote sensing activities to be conducted with the EAARL system. A USGS lab will be set up for lidar work at the Salisbury Airport Hangar in Salisbury, Maryland.  Airborne lidar operations for USGS CMG projects will be based in Salisbury, thereby minimizing any disruption of operations during the hurricane season in Florida and the NGOM region. Currently available USGS aircraft will also be assessed for possible future deployment of the EAARL sensor.

Hardware and software enhancements will be made to the EAARL sensor to improve depth penetration capabilities at the same spatial and temporal resolution. These enhancements include investigation of methods to improve the laser transmitter and optical receiver on the EAARL system to advance daylight operations, depth penetration, and allow multiple fields of view.

EAARL lidar surveys will be planned and conducted in the NGOM region to fill in data gaps and site locations that require additional information not available from currently available data sets. Example locations include mapping of the Atchafalaya Basin and the coastal floating marsh and wetlands in Louisiana. The acquired data will be processed to create GIS-ready Digital Elevation Model data sets at the highest spatial resolution. Additional research and algorithms will be required to derive topography under the float-on marsh and along the land-water interface.