Yosemite National Park High-Resolution Digital Topography Data Collection

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The purpose of this project is to collect high-resolution digital topography data from five park areas. Data would be collected by aircraft using airborne light swath mapping (ALSM), often referred to as LIDAR. The proposed flight areas are the Mariposa Grove (3.0 km2), Yosemite Valley (72.2 km2), El Portal (4.5 km2), and Tuolumne Meadows and Glen Aulin (16.0 km2). High-resolution digital topography for these areas would serve many purposes, among them aiding the placement of roads and trails in the Mariposa Grove, flood plain mapping in support of the Tuolumne River Wild and Scenic River Plan and Tuolumne Meadows Development Concept plan, and a number of other management and planning activities.


Similar to radar technology (which uses radio waves instead of light), ALSM determines the distance to an object by measuring the time delay between a pulse of light and detection of the reflected signal. ALSM data are collected using small twin-engine aircraft. The plane flies 2000 ft above the ground surface at 100 mph, using ground-based GPS units to orient. A laser mounted on the plane directs pulses of light toward the ground. The light is scattered back to a sensor in the aircraft, and the round trip travel time is used to compute the precise three-dimensional locations of points on the ground. Resulting digital topographic maps have ~0.5 m accuracy, a dramatic improvement over the 10 m topography presently available. Buildings, trees, and boulders are individually discernable features.

ALSM data would be collected by the National Center for Airborne Laser Mapping (NCALM), a National Science Foundation (NSF)-sponsored institution based out of the University of Florida. Because NCALM will be in the area collecting data for an NSF-funded researcher, the mobilization costs are subsidized, resulting in SIGNIFICANTLY decreased costs for the park. The project would be staged out of local airports outside of the park. NCALM would provide personnel and GPS receivers to operate ground stations, as well as a pilot, laser operator, and aircraft. The project manager would assist in this effort.


Four of the five proposed flight areas can be flown at or above the minimum flight ceiling of 2000 ft. However, in Yosemite Valley aircraft are required to stay 2000 ft above the Valley rim, which equates to roughly 5000 ft above the Valley floor. This distance is too high to collect precise ALSM data, so a categorical exclusion is necessary to collect data in Yosemite Valley.


Flight durations would depend on the size of the areas (see above); data collection for Yosemite Valley, the largest area, would likely take three to five hours. Visitors might experience increased noise (in the form of airplane engine noise) and decreased vista quality (in the form of the airplane in the sky) during this time. The laser used for mapping is not detectable to the naked eye and does not pose a threat to human health.

One important issue concerns peregrine falcons. Aircraft are required to maintain 0.5 mile line-of-sight distance from peregrine nests until August 1 or when peregrine chicks have fledged, whichever is later. The flights would occur after August 1, but until then Resource personnel would monitor a nest in western Yosemite Valley that started late. Consultation with Wildlife Biologists would ensure peregrines are not affected. If the chicks have not fledged by the planned flight time, the aircraft would not fly within 0.5 mi of this nest.

Park Dispatch, FAA, Fire Officers, Law Enforcement, and others would be informed of the flights well ahead of time, and consulted to coordinate flights. The park PIO and Interpreters would be notified of the flights so they may help convey information about the flights to concerned visitors.

There are presently no other means of collecting high-resolution digital topography data except by aircraft.