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NOAA's REMOTE SENSING ACTIVITIES
January 8, 2002 Remote sensing is the science of remotely acquiring, processing, interpreting and presenting spatial data for objects and environmental processes using signals from a broad range within the electromagnetic spectrum. Remote sensing instruments are able to produce images of the physical properties and characteristics of objects without being in physical contact with them. Instead, this highly advanced technology forms images by gathering, focusing, and recording reflected light from the sun, energy emitted by the object itself, or reflected radar waves (which were emitted by the satellite or other remote sensing devices). Therefore, remote sensing can be further characterized as either "passive" and "active." Passive remote sensing detects available (background) electromagnetic energy from natural sources (such as sunlight), while active remote sensing, depends on an artificial "light" source (such as radar) to illuminate the scene.
NOAA's aircraft and satellites are common platforms from which the NOAA's remote sensing observations are made. For most photographic missions, NOAA deploys a Cessna Citation II aircraft equipped with dual camera capability. This dual capability allows two types of images to be collected concurrently (if conditions allow), a feature which is critical to many of NOAA's remote sensing activities. NOAA also uses commercially available one meter (high resolution) Space Imaging IKONOS imagery for shoreline change analysis.
NOAA uses digital photogrammetric work stations to produce numerous remote sensing products, including shoreline vectors and maps, digital elevation models, digital terrain models, airport layout diagrams, aviation obstruction charts, and various other special use maps and products.
REMOTE SENSING TECHNIQUES USED BY NOAA
Since the late 1930s, precision aerial photography has been the primary data source for coastal survey maps, shoreline feature delineation maps, nautical charts and other agency coastal geographical information systems. Unfortunately, however, aerial photography has limitations in that it can only provide high resolution spacial imagery when weather (e.g., cloud cover, sun angle) and environmental (e.g., tidal) conditions are optimal. Furthermore, the spectral sensitivity of aerial photography is limited to small range from about near ultraviolet to near infrared. Therefore, NOAA is currently investigating existing and new remote sensing technology to augment and/or replace conventional aerial photgrammetry. Unlike traditional aerial photography, these techniques are able to capture images derived from a much broader portion of the electromagnetic spectrum (from low-frequency radio waves through gamma-ray regions of the spectrum) and in some cases are not restricted by time of day, weather conditions, and other environmental anomalies. Furthermore, more advanced remote sensing technologies can be less expensive than collecting the same type and quantity of data using aerial photography and conventional ground survey techniques. Currently, NOAA is using and investigating several different remote sensing technologies, including the following:
Aerial Photography: Taking aerial photographs of the Earth's surface is a passive form of remote sensing generated from cameras mounted on aircraft, satellites and other spacecraft. The photographs are taken every 10 to 30 seconds as an aircraft follows a systematic overlapping flight pattern at a fixed altitude. Each picture slightly overlaps the preceding picture so that a stereoscopic (3-D) image of the entire area can be produced and ground objects can be more easily interpreted (note that a strict set of mathematical corrections are applied in an aerotriangulation process prior to compilation to remove errors such as atmospheric refraction, film shrinkage, and underwater refraction). NOAA's primary aerial photographic product is a 9x9 inch color photograph, which is usually exposed at scales from 1:10,000 to 1:50,000. NOAA photographers can also capture images from select parts of the electromagnetic spectrum by using various combinations of films and filters. The types of imagery they usually collect include natural color, panchromatic (black-and-white), false-color infrared, and black-and-white infrared photography.
OTHER RELATED REMOTE SENSING APPLICATIONS
Light Detection and Ranging: LIDAR is one type of advanced remote sensing technology currently being used by NOAA. LIDAR is an active laser remote sensing system that can be operated in either a profiling or scanning mode using pulses of light to illuminate the terrain. LIDAR data collection involves mounting an airborne laser scanning system onboard an aircraft along with a Global Positioning System (GPS) receiver (to locate x, y, and z positions of the sensor) and an inertial navigation system (to provide pitch and roll of the aircraft and sensor attitude information). Laser pulses, up to 33,000/sec., are transmitted and received by the LIDAR instrument. By accurately measuring the round trip travel time of the laser pulse from the aircraft to the ground, highly accurate spot elevations can be calculated. Depending upon the altitude and speed of the aircraft and the laser repetition rate, it is possible to obtain point densities (laser pulses on the ground) that would likely take months to collect using traditional aerial photography and ground survey techniques. Furthermore, LIDAR data is collected digitally, so the interpretation errors sometimes associated with more traditional elevation data compilation methods are eliminated. NOAA has used LIDAR to assess post storm damage to beaches, cities and building structures; measure heights within forest timber stands; and airspace obstruction for the Federal Aviation Administration. NOAA recently used LIDAR to map the World Trade Center and the Pentagon after the Sept. 11 tragedy.
Imaging Spectroscopy: The use of imaging spectrometers is a passive form of remote sensing, which gathers data over a wide band of the electromagnetic spectrum via many small bandpass channels. Such data (often called hyperspectral data) can be used to accurately determine the composition of the earth below. When the images are acquired at high spatial resolution, the resulting data provides a robust characterization of the earth's surface.
Synthetic Aperture Radar: SAR is an active remote sensing technology, which uses microwave electromagnetic energy to form complex images of terrain reflectivity. Because SAR is largely unaffected by the presence of dense cloud cover, its use is uninhibited by weather constraints and can be used for both daytime and nighttime operations. SAR systems take advantage of the long-range propagation characteristics of radar signals and the complex information processing capability of modern digital electronics to provide extremely high resolution imagery, which is uniquely different from the reflectivity measured in optical and IR imagers. SAR imagery is often described best as a representation of the reflecting properties (backscatter) of the terrain, which changes depending on the physical properties of the surface, surface texture, size and orientation, as well as the microwave reflectivity of the individual ground components. NOAA has been researching and utilizing SAR technology to derive shoreline in Alaska for NOAA Nautical charting efforts for the past four years. It has shown great promise in this area of the country, where it has been very difficult to map in the past due to weather and geographic remoteness.
NOAA has also been able to combine several remote sensing techniques to create a variety of other products. Specifically, aerial photographs and other remote sensing imagery can be merged to produce contour maps, digital orthophotos, and three-dimensional models of a surface area through a process called digital photogrammetry. The image to the left, for example, is a shaded relief map generated using a surface elevation model to depict vegetation heights.
Likewise, the advent of high resolution space borne imagery may be useful for precision mapping projects. NOAA has recently used different commercial space borne imagery as a reconnaissance tool to evaluate the temporal accuracy of previously compiled shoreline. The image to the left, for example, is an old remote sensing image of Hooker's Point in Tampa, Fla., while the red line superimposed on top of the image depicts a more recent shoreline (that has obviously retreated in recent years). Likewise, the blue and yellow lines represent other recent changes in shoreline and alongshore features detected using other remote sensing techniques.
REMOTE SENSING SUPPORTS NOAA PROGRAMS
NOAA plans, coordinates, monitors and provides technical direction for all types of programs requiring the use of aerial photographs and remotely sensed data. Such programs and activities include:
Airport Obstruction Charting: NOAA uses remote sensing to create Airport Obstruction Charts, 1:12,000 scale graphics depicting "FAR Part 77, Objects Affecting Navigable Airspace surfaces," which are digital images representing objects that may disrupt critical airspace, aircraft movement and apron areas (i.e., the area near the terminals). AOCs are important in that they provide data for: computing maximum take-off and landing weights, establishing instrument approach and departure procedures, and completing engineering studies pertaining to obstruction clearance and airport facility improvements. NOAA is also evaluating several advanced remote sensing technologies (including digital photogrammetry, LIDAR, SAR, and imaging spectroscopy) for possible integration into the airport obstruction charting program. In the future, for example, remote sensing data may be used to create digital surface models to analyze airport obstructions. NOAA is currently conducting research at the Asheville (N.C.) Regional Airport, Gainesville (Fla.) Airport and Harrisburg (Pa.) International Airport with hyperspectral, LIDAR, and digital photogrammetric data.
Coastal Mapping: NOAA uses remote sensing technologies (primarily aerial photography) to survey the nearly 95,000 miles of U.S. shorelines. These surveys provide data for the production of nautical charts and geographical references needed for managing coastal resources. Aerial photography surveys are flown on varying time cycles to update the data periodically depending on the amount of change caused by man-made or natural forces.
Boundary Determination: NOAA uses remote sensing to delineate the mean lower low water (MLLW) (or MLW in some states) and mean high water (MHW) lines used in boundary determination. Specifically, black and white infrared photography provides a sharp contrast between water and land especially along gentle sloping shore areas. The accurate location of shoreline boundaries is extremely important because it is the boundary that defines private, state, and federal ownership.
Coastal Wetlands Mapping: Mapping and monitoring of wetlands over large areas is greatly facilitated by the use of remotely-sensed imagery. NOAA is analyzing multi-seasonal SAR imagery, in conjunction with Landsat and other data holdings, with the aim of assessing the potential for improved characterization and delineation of wetlands.
Benthic Mapping: NOAA's benthic mapping project is a cooperative state and federal effort to map benthic resources (i.e., those which reside at bottom of a water body) throughout the U.S. coastal regions. The focus of this project are living resources in the near-shore estuarine and marine environments including seagrass meadows, coral reefs, hard bottom areas, shellfish beds, and algal communities. The primary goal of the program is to establish an ongoing and consistent national database of coastal benthic data to document changes and trends over time. The project relies primarily on high-resolution metric aerial photography and photogrammetry to build the national database.
Photogeodesy and Photobathymetry: NOAA uses photogrammetry to map offshore and underwater features by measuring the depths of moderately clear, near-shore coastal waters and lakes from a low-altitude aircraft using a scanning, pulsed laser beam. These maps are also used to monitor coastal structures, track coastal substrate, and assist with coastal protection, resource management and exploitation. Although turbid water often interferes with this remote sensing application, it is possible to obtain remote sensing imagery down to depths as great as 50 meters or more, in clear coastal waters. The image below, for example, shows bathymetric contours spanning the northern Gulf of Mexico and northern Bahamas at a scale of approximately 1:2,100,000.
Relevant Web Sites
NOAA REMOTE SENSING EXPERTISE AIDS WORLD TRADE CENTER RECOVERY EFFORTS
COMMERCIAL SATELLITE LICENSING INFORMATION AVAILABLE ON LINE, NOAA ANNOUNCES
Current Status: Coastal Aerial Photography
Photographic prints from about 1945 to the present
NOS MapFinder (contains a complete index of about 41,000 aerial photographs taken since 1990)
Greg Hernandez, NOAA, (202) 482-3091