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Remote-Sensing Technologies Provide Unique Maps and Datasets to Support Coastal Scientists, Managers, and Decision Makers
The U.S. Geological Survey (USGS) Coastal and Marine Geology Program develops and uses specialized technology to build high-resolution topographic and habitat maps. These data products are critical to researchers, decision makers, resource managers, planners, and the public. A new Web site launched by the program, "Decision Support for Coastal Science and Management," provides data products and details about the capabilities of various USGS remote-sensing technologies. The information and tools provided by the Web site support research and decision making regarding important resources and vital habitat in coastal and marine environments.
"There are so many different applications of this technology that are relevant to the day-to-day life of the public," said Amar Nayegandhi, computer scientist and program manager of the decision-support Web site. A topobathymetric map of Tampa Bay is one example.
"Lidar [light detection and ranging] technology, along with other remote-sensing applications, was used to create a topographic map of Tampa Bay and the surrounding area that was extremely popular because it allowed people to identify the elevation of their property," said Nayegandhi. The value of detailed up-to-date topographic data to coastal managers and emergency planners in low-lying coastal areas is obvious; many regions at or near present sea level are vulnerable to flooding from hurricanes and other storms.
High-resolution maps of topography, bathymetry, and habitat describe important features affected by coastal-management decisions. The mapped information serves as a baseline for evaluating resources and tracking the effectiveness of resource- and conservation-management decisions. Datasets can be used to describe the topographic complexity of a barrier island or to monitor changes caused by extreme storms.
"Topography is so fundamental because it provides a dataset baseline—a context wherein you can understand all other resources—which is of prime importance to natural- and coastal-resource managers," said John Brock, oceanographer and principal investigator of the Decision Support Project.
Examples of recent lidar projects include surveying various characteristics of the benthic habitat (seafloor) of the Florida reef tract and Dry Tortugas National Park and mapping coastal changes in barrier islands on the U.S. Atlantic and Gulf of Mexico coasts, as well as along other coastal areas nationwide.
"On the coastal side, we've used this technology to try to understand how beaches and coastlines respond to hurricanes by accurately measuring the volume of sand and land that has been lost during a storm," said Nayegandhi.
The remote-sensing lidar tool known as EAARL (Experimental Advanced Airborne Research Lidar) uses laser technology to collect highly detailed elevation information that is processed by using a custom-built Airborne Lidar Processing System (ALPS). The EAARL system is unique in that it can simultaneously map elevations of both the land and the seafloor in clear shallow waters. The lidar data can also be processed to reveal the detailed structure of other features, such as buildings, trees, or coral reefs. In addition to mapping coastal changes and shallow-marine habitats, lidar has been used on land to map vegetation canopies, determine change in canopy structure, estimate volumes of vegetation available as wildfire fuel, and provide digital-elevation imagery for resource managers.
"It also gives us seabed topography, and that allows us to determine how healthy coral reefs are by measuring the rugosity or topographic roughness," said Nayegandhi. Generally, the rugosity of a coral reef can be used to assess coral-reef health.
The USGS uses a specialized aircraft, the Pilatus Porter model PC6T, to collect these types of data. All but three seats have been removed from the normally eight-passenger aircraft, and a large, downward-looking port has been installed in the floor. Owned and operated by the USGS since 1975, the aircraft was transferred to the lidar project in 2008. The lidar is mounted above the port, giving it an unrestricted downward view of the Earth.
The crew consists of a pilot and a lidar operator who work together to operate and monitor the instrumentation onboard the aircraft. The pilot uses the navigation equipment to carefully guide the aircraft over the desired targets.
"The plane is uniquely designed to acquire very high resolution images by being able to fly low, around 300 m, and slow, between 60 and 90 knots, over areas of interest," said Wayne Wright, physical scientist at the USGS. "The plane is instrumented with 10 onboard networked computers that work together to capture the lidar, photography, navigation, and orientation data required to produce the high-accuracy remotely sensed datasets."
The laser repetitively emits a short (1.5 nanosecond) pulse of light that travels from the onboard sensor to the ground and is then reflected back up to the sensor. By timing how long it takes each pulse to reach the ground and return, scientists get a highly accurate measure of the distance between the sensor in the aircraft and the ground. The reflected light is then converted to digital values and passed to the onboard computer, where the information is stored.
Although the laser is used to precisely measure the distance from the airborne sensor to the ground, more information is required before maps can be made. Onboard survey-grade global-positioning-system (GPS) receivers record the precise position of the onboard sensor in three-dimensional space. Once this information is acquired, it can be combined with the distance measurement from the laser to compute the elevation of the surface and the location where the laser pulse contacted the Earth.
Lidar technology allows the USGS to create a whole suite of products ranging from topographic maps to digital-elevation models. Projects such as Decision Support for Coastal Science and Management are examples of how the USGS is uniquely suited to serve various partners and resource managers with new technologies that provide fundamental information to answer today's resource questions and challenges.About the author: Matthew Cimitile is a journalist working with the USGS Florida Integrated Science Center office in St. Petersburg.
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Remote Sensing Provides Unique Maps and Datasets
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