To collect LIDAR data, the NOAA/USGS/NASA partnership uses a NASA-engineered laser ranging system called the Airborne Topographic Mapper (ATM). This system is mounted on board an aircraft, typically a NOAA DeHavilland Twin Otter or a NASA Lockheed P-3B. Once in flight, the aircraft travels over the ground at approximately 130 miles per hour (60 meters per second). During the flight, the LIDAR sensor pulses a narrow, high frequency laser beam toward the earth through a port opening in the bottom of the aircraft's fuselage. The LIDAR sensor records the time difference between the emission of the laser beam and the return of the reflected laser signal to the aircraft.
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The laser itself cannot move. However, there is a small folding scan mirror mounted on the LIDAR sensing instrument that can be rotated. The mirror has a 30-degree angle field of view and can be moved so that the laser beam can take elevation measurements in an elliptical area approximately 300 meters wide. View animated displays of the data collection process. |
The ATM calculates elevations by measuring the time taken for laser pulses to leave the sensor, reflect off of the ground, and return to the sensor. Simultaneously, the position of the plane is measured with a differentially corrected Global Positioning System (GPS) receiver, and the attitude (roll, pitch, and yaw) is measured with an inertial navigation system. Once the position and attitude of the plane are known, the position of the pulse reflection on the ground can be calculated. The end product is accurate, geographically registered longitude, latitude, and elevation (x,y,z) positions for every data point. These "x,y,z" data points allow the generation of a digital elevation model (DEM) of the ground surface.
LIDAR flights are planned to maximize the number of elevation points collected at the lowest tide for the largest area possible. The aircraft flight path is generally parallel to the beach. Four passes are flown over each section of the beach. Two of these passes are flown so the center of the swath is over the sand/water interface. The other two passes are flown over the center of the sand/development interface.
Flights generally last four hours, and weather conditions must be monitored. The flights cannot be flown during times of rain or fog as the water vapor in the air could cause the laser beams to scatter and give a false reading. Additionally, the plane cannot fly during times of high winds as the returned laser pulse will not be recorded correctly.
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