Airborne Lidar Sensing of Coral Reef Topographic Complexity in Biscayne
National Park, Florida
John C. Brock, Tonya D. Clayton, and Amar Nayegandhi
U.S. Geological Survey, Center for Coastal and Watershed Studies, St.
Petersburg, FL
C. Wayne Wright
NASA Goddard Space Flight Center, Wallops Flight Facility, Wallops Island, VA
Variability in vertical relief, or rugosity, combined with diversity of substratum
type, creates habitat complexity, a factor that both reflects and governs the spatial
distribution and density of many reef organisms. The NASA Experimental
Advanced Airborne Research Lidar (EAARL) is designed to measure the
topographic complexity of shallow reef substrates. The EAARL is a narrow beam
divergence, high pulse repetition frequency, temporal waveform-resolving,
airborne green wavelength lidar (TWRAGL) mounted on an aircraft platform
whose position is determined by carrier phase differential GPS techniques. This
unique instrument has cross-environment coastal surveying capabilities over
beaches, vegetated land, and especially, the clear water settings of most shallow
coral reefs.
During early August 2002, 8 EAARL flights were conducted over the northern
Florida Keys reef tract. EAARL lidar coverage was acquired by at least 2 flight
passes over all of a broad swath extending from north of Turtle Reef to south of
Carysfort Reef. In order to insure acquisition of dense lidar coverage for optical
rugosity analysis of reefs in Biscayne National Park, the August 4, 2003 and
August 5, 2003 missions each repeated a single flightline positioned to survey
several bank reefs and numerous patch reefs, respectively.
Five sites selected for optical rugosity analysis (ORA) match sites of recent field
monitoring of coral cover and species richness. The remainder of the 15 ORA
sites were selected based on an inspection of recently acquired AISA airborne
hyperspectral and QuickBird satellite images. Lidar temporal waveform data sets
were extracted for the ORA sites, and processed to create a voluminous set of
submarine topographic transects for each site referenced to the NAVD88 vertical
datum and the NAD83 horizontal datum. This step involved the use of algorithms
that examine each lidar temporal waveform to determine the range to the sea
surface and the water column thickness.
Each lidar topographic transect was evaluated separately to determine at each
point the direct geometric distance (Dgeom) to the transect origin point, the
transect-following distance (Dtran) to the transect origin point, and the elevation
difference relative to the adjacent point closer to the transect origin point (Eap).
At each point, partial transect optical rugosity (Ropart) was calculated as:
Ro part 
Dtran
Dgeom
The optical rugosity assigned to the entire transect (Rotran) was set equal to the
peak partial transect rugosity obtained for that transect :
Rotran  max Ropart
In addition, the average and maximum adjacent point elevation change, Av(Eap)
and Mx(Eap), respectively, were calculated for each transect. Finally, the wholesite mean and maximum values of Rotran and Av(Eap) for the entire population of
transects at each ORA site, along with their standard deviations, were calculated.
The optical rugosity analysis revealed a distinct geographic pattern of variation in
whole-site measures of TWRAGL-based topographic complexity that matches
previous in situ assessments of the relative habitat complexity of bank reefs and
patch reefs in the northern Florida Keys reef tract. Three groupings of sites,
Pacific Reef sites, Ajax Reef sites, and patch reef sites, are clearly evident along a
curvi-linear trend towards increasing topographic complexity on a plot of wholesite mean Rotran versus site mean Av(Eap) (Figure 1). This result is consistent
with field observations of higher stony coral cover, octocoral cover, and coral
species richness on patch reefs relative to shallow bank reefs within the study
area. Further, both the north Pacific Reef sites and the south Ajax Reef sites form
tight clusters, revealing subtle but highly reproducible morphological differences
between these quite similar bank reef regions. An analogous pattern of variation
between bank and patch reefs is evident on a plot of the standard deviation of
whole-site mean Rotran versus the standard deviation of whole-site mean Av(Eap)
(Figure 2). Both the north Pacific Reef and south Ajax Reef sites form tight
clusters at low relative standard deviation of both measures of topographic
complexity, with variability slightly higher at Ajax Reef. All but one of the patch
reef sites fall along a rough trend line toward higher internal-site variability in
both Rotran and Av(Eap).
Figure 1.
Figure 2.