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June 2020
PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING
PHOTOGRAMMETRIC
ENGINEERING &
REMOTE SENSING
J
ournal
S
taff
Publisher ASPRS
Editor-In-Chief Alper Yilmaz
Assistant Editor Jie Shan
Assistant Director — Publications Rae Kelley
Electronic Publications Manager/Graphic Artist
Matthew Austin
Photogrammetric Engineering & Remote Sensing
is the official journal
of the American Society for Photogrammetry and Remote Sensing. It is
devoted to the exchange of ideas and information about the applications of
photogrammetry, remote sensing, and geographic information systems. The
technical activities of the Society are conducted through the following Technical
Divisions: Geographic Information Systems, Photogrammetric Applications,
Lidar, Primary Data Acquisition, Professional Practice, and Remote Sensing
Applications. Additional information on the functioning of the Technical
Divisions and the Society can be found in the Yearbook issue of
PE&RS.
Correspondence relating to all business and editorial matters pertaining
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Copyright by the American Society for Photogrammetry
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When oceanographer Serge
Andréfouet first saw a satellite
image of the Great Bahama Bank,
he knew the colors and contours
were special. He passed the
unique image to a colleague, who
submitted it to NASA’s Earth Ob-
servatory (EO) for an Image of the
Day in 2002 (top image). Nearly
eighteen years later, the image is
still much appreciated. In fact, it
knocked off more recent satellite imagery to win EO’s Tournament Earth 2020.
“There are many nice seagrass and sand patterns worldwide, but none like this
anywhere on Earth,” said Andréfouet, who is now studying reefs at the Institute for
Marine Research & Observation in Indonesia. “I am not surprised it is still a favorite,
especially for people who see it for the first time.” He said the image has been fea-
tured over the years on numerous websites, in books, and even at rave parties.
The varying colors and curves remind us of graceful strokes on a painting, but the
features were sculpted by geologic processes and ocean creatures. The Great Bahama
Bank was dry land during past ice ages, but it slowly submerged as sea levels rose.
Today, the bank is covered by water, though it can be as shallow as two meters
(seven feet) deep in places. The bank itself is composed of white carbonate sand and
limestone, mainly from the skeletal fragments of corals. The Florida peninsula was
built from similar deposits.
Andréfouet’s image (above) shows a small section of the bank as it appeared on January
17, 2001, and was acquired by the Enhanced Thematic Mapper Plus (ETM+) on the
Landsat 7 satellite (using bands 1-2-3). At that time the instrument’s blue channel (band
1) helped distinguish shallow water features better than previous satellite mission.
The wave-shaped ripples in the images are sand on the seafloor. The curves follow the
slopes of underwater dunes, which were probably shaped by a fairly strong current
near the sea bottom. Sand and seagrass are present in different quantities and at dif-
ferent depths, which gives the image a range of blues and greens. The area appeared
largely the same when Landsat 8 passed over on February 15, 2020 (cover image).
The shallow bank quickly
drops off into a deep,
dark region known as the
“Tongue of the Ocean.”
Diving about 2,000 meters
(6,500 feet) deep, the
Tongue of the Ocean is
home to more than 160 fish
and coral species. It lies ad-
jacent to the Andros Island,
the largest in the Bahamas
and one of the largest
fringing reefs in the world.
The image to the right was
acquired on April 4, 2020, by the Moderate Resolution Imaging Spectroradiometer
(MODIS) on NASA’s Terra satellite.
At the time of the 2001 image, researchers did not have a good understanding of the
location and distribution of reef systems across the world. Global maps of coral reefs
had not changed much since the 19th Century. So researchers turned to satellites for a
better view. Andréfouet’s image was collected as part of the NASA-funded Millennium
Coral Reef Mapping Project, which aimed to image and map coral reefs worldwide.
The project gathered more than 1,700 images with Landsat 7, the first Landsat to take
images over coastal waters and the open ocean.
Today, many satellites and research programs continue to map and monitor coral reef
systems, and marine scientists have a better idea of where the reefs are and how they
are faring. Researchers now use reef images and maps in tandem with sea surface
temperature data to identify areas vulnerable to coral bleaching.
For more information or to see all three images, visit
.
gov/view.php?id=146697.
NASA Earth Observatory images by Joshua Stevens, using Landsat data from the
U.S. Geological Survey, and MODIS data from NASA EOSDIS/LANCE and GIBS/Worl-
dview.2002 imagery courtesy Serge Andrefouet, University of South Florida. Story by
Kasha Patel.
