Recent Advances in Lidar Remote Sensing: Applications to Geer

High-Resolution, Small-Footprint, Waveform-Resolving Lidar: EAARL Applications in the Florida Keys Reef Tract

T. Clayton, J. Brock, A. Nayegandhi U.S. Geological Survey, St. Petersburg FL USA 33701

C.W. Wright NASA Goddard Space Flight Center Wallops Island, VA USA 23337

The term ‘lidar’ (derived from ‘light detection and ranging’) refers to active optical techniques that use a pulse of laser light to make range-resolved remote measurements. Distance between the lidar sensor and reflecting target(s) is calculated as a function of time elapsed between transmission of a well- characterized laser pulse and its return to the detector (i.e., the two-way travel time), and the speed of light in the medium of transmission. Such techniques are finding wide application in the fields of topographic and hydrographic surveying, forestry, and the geological and atmospheric sciences.

A number of South Florida lidar coastal applications are now being explored with the NASA Experimental Advanced Airborne Research Lidar (EAARL), a relatively lightweight, low-power sensor designed for deployment on light aircraft. The system is presently capable of synchronized collection of precision navigation data, georectified digital aerial photography, and temporally resolved lidar waveform data. Coupled hyperspectral imaging capabilities are to be added early in 2003. Designed for cross-environment (subaerial and subaqueous) applications, the lidar component of EAARL utilizes a green (532-nm) laser for maximum water penetration. Under typical surveying conditions (300-m operating altitude, two-pass coverage), EAARL covers approximately 43 km2/hr, with a swath width of ~240 m, a spot size (“footprint”) of ~15 cm and horizontal sample spacing of ~ 1 x 1 m. Each returned laser waveform is sampled every 1 ns, which is equivalent to, vertically, every 15 cm in air and 11 cm in water.

Because of the unique nature of the system, off-the-shelf software is not available for EAARL data processing, feature extraction, and image display. The development of new software has necessarily accompanied the design, construction and deployment of the hardware components. The processing code is written in Yorick, a high-level programming language designed for scientific applications, while the integration platform and graphical user interface (GUI) capabilities have been built using the Tcl/Tk language and toolkit. The raw EAARL waveform data are processed to return derived data in the form of first- return elevation (e.g., top of vegetation in subaerial vegetated areas), bald-earth elevation, water-surface elevation, submerged topography, and water depth. For the purposes of data exploration and display, the software package includes the capability to display and query linked aerial photographs, individual returned waveforms, composite rasters, and maps. A second custom software package utilizes the aircraft navigation data and camera parameters to generate georectified photomosaics of the digital aerial photography collected during each lidar survey.

EAARL capabilities and their potential applications are presently being explored in a variety of environments, including the optically transmissive waters of the Florida Keys reef tract. EAARL laser returns in these waters are retrieved typically from < 15 m water depth. Including engineering test flights, three surveys have been conducted in South Florida (July 2001, September 2001, and August 2002), covering portions of Biscayne and Dry Tortugas National Parks, as well as the northernmost reaches of the Florida Keys National Marine Sanctuary.

In all, approximately 160 GB of laser sounding data and 200,000 accompanying digital aerial photographs have been collected over the Florida reef tract. The lidar data are now being processed at the U.S. Geological Survey, with the aim being the routine production of easily extractable data subsets as well as GIS- ready point data, grids, and maps. After post-processing of the associated GPS data, the EAARL lidar data are parsed into 2 x 2 km tiles for ease of management. Present geographic coverage over the Keys includes 360 of these tiles, grouped into 40 larger (24 x 24 km) index tiles. Each tile and index tile is named according to the easting and northing values of its northwestern corner, as are any features that are extracted during subsequent analysis (e.g., individual patch reefs). Each flightday’s data is first processed tile-by-tile for initial checks on data quality and internal consistency. Subsequently, all data for a given tile are merged and used to create final map products and provide input data for subsequent analyses.

Initial EAARL surveys have been targeted to satisfy not only basic research needs, but also high-priority areas identified by those with stewardship responsibilities for the reefs. One direct outcome from the EAARL surveys is the production of high-resolution topographic/bathymetric maps. Such maps are of obvious utility for field researchers, resource managers, and recreational users. A particular advantage of this type of digital data is the potential for automated feature extraction. Biscayne National Park, for example, contains thousands of small patch reefs inshore of the bank-edge barrier reefs. While, due to their sheer numbers, characterizing or even identifying each one manually would be a Herculean task, the EAARL data lends itself well to this type of problem.

Another potential application of this high-resolution topographic (subaerial and submerged) data is the provision of boundary conditions for numerical models of circulation, sediment transport, wave run-up, and tsunami and hurricane inundation. Earlier data sets for many areas of South Florida and the Caribbean are inadequate for such applications.

One novel application takes advantage of the unique capabilities afforded by the small footprint and high-resolution sample spacing of EAARL. Preliminary analyses indicate that EAARL-derived “optical rugosity” of the patch and barrier reefs of the northern Keys may provide a meaningful landscape measure related to habitat complexity.

Other research topics and applications currently being explored with EAARL data collected in the South Florida region include ongoing algorithm development and refinement, as well as pilot inquiries into lidar characterization of water-column and subaerial vegetation/canopy vertical structure.

Future work will be aimed at expanding both the geographic coverage and site diversity of the EAARL coral reef data set. If adequate funding partnerships can be established, additional EAARL surveys will be flown to cover the Middle and Lower Keys as well as selected sites in the Caribbean. Surveying the remainder of the Keys reef tract could lead to a seamless series of high-resolution, GIS-ready map products. Surveying sites outside the Keys region would provide the expanded range of site diversity that is required to more thoroughly explore EAARL capabilities in terms of substrate characterization.

Clayton, Tonya, U.S. Geological Survey, Center for Coastal and Watershed Studies, 600 4th St. S., St. Petersburg, FL, 33701 Phone: 727-803-8747, Fax: 727-803-2031, [email protected]`