Mapping Alaska's Changing Parks from a Plane: The NPS Aerial Camera System

The Denali Park Road, which most of the park's roughly 600,000 visitors in 2017 traveled by tour bus, is sliding. In 2018, where the road crosses the Pretty Rocks landslide, it subsided by up to 1.0 meter in three months, and National Park Service (NPS) staff detected it by flying a plane with two cameras overhead in June and September. That is aerial structure from motion (SfM), a technique the NPS Alaska Region has used since 2017 to map and monitor landscape change across parks too large and remote to survey on foot. The method was described by NPS staff in Alaska Park Science.

The Kennecott area in Wrangell-St. Elias National Park and Preserve: orthophoto mosaic at 16 cm resolution (left) and digital elevation map at 32 cm resolution (right) (images from NPS).

A gap between satellites and drones

SfM uses algorithms and many overlapping photographs to solve for the camera's position and the 3D shape of the scene, so ordinary consumer cameras can replace the precisely engineered ones of traditional photogrammetry. It yields orthophoto mosaics and digital elevation models (DEMs). The Alaska system, built with help from the U.S. Fish and Wildlife Service and the Bureau of Land Management, has two cameras, one red-green-blue and one near infrared, and survey-grade GNSS units. Pilots follow gridded flight lines, and each shutter release is logged by the positioning unit. A typical day covers about 100-300 km2: Anchorage, at 250 km2, could be mapped at 15 cm resolution in about five hours of flying.

How it has fared

Weather is the main constraint.

*Doubled when the near-infrared photos are counted.

Aircraft GNSS positions, corrected against a base station ideally within 50 km, are accurate to about 5 cm (90%), which puts the cameras within about 15 cm. The resulting orthophoto mosaics accurate to about ±15 cm (95%) and DEMs with a precision of about ±10 cm (95%). Those DEMs sit about 25 to 70 cm lower than the ground, so where vertical accuracy matters, ground control points correct them. Repeat flights can detect changes of about 20 cm or more. One limit is vegetation: unlike LiDAR, SfM cannot measure the ground under thick plants. On bare ground or ice, though, it matches LiDAR, as on Bear Glacier in Kenai Fjords National Park.

What it has shown

Applications include coastal erosion, intertidal and salt-marsh mapping, landslides, mining and abandoned mineral lands, river restoration planning, archeological site maps, fire and permafrost changes, glacier retreat and glacier outburst floods.

Exit Glacier, Kenai Fjords. One of the most visited land-terminating glaciers in North America is becoming harder to reach: its toe is now in a cliff-sided gorge. SfM has been flown there every year since 2016 to measure the volume change, and subtracting a 2008 LiDAR DEM from a 2018 SfM DEM shows the loss. The Harding Icefield area dropped 10 to 20 meters vertically, 100 meters of ice melted from the glacier's toe, and the net loss is 12.6 m of ice, an annual average of 1.26 m. The high-elevation index sites show a slight gain to no change and the low ones a net loss, so the icefield loses mass from its outlet glaciers faster than it gains snow.

The Arctic coast. Bering Land Bridge National Preserve and Cape Krusenstern National Monument hold about 1,600 km of soft-sediment Arctic coast, with growing ship traffic through the Bering Strait and a deep-water port planned at Nome. Comparing 2018 SfM with 2004 NOAA LiDAR shows dune ridges moving inland, with crests shifted a few tens of meters south-southwest. For landcover, the team compared satellite classification at 5 m with SfM at 20 cm using all four spectral bands, NDVI, DEM, topographic position index and a random-forest classifier. The early results are encouraging, and the maps can serve as a basemap for oil spill planning.

Denali Park Road. In 2017 the park had about 600,000 visitors, most of them on the road, and as of 2018 about 30 unstable slopes were known along the Polychrome section (miles 44 to 46). Thawing permafrost is increasing landslides. The Pretty Rocks landslide moves about 8 cm a day where the road crosses it. Flights on June 6 and September 27, 2018 showed the road slumping and failures within the slide, and toggling between the two DEMs shows the movement that a vertical-change map misses.

Looking ahead

Demand for the system grew over its first four years. Its greatest challenge is meeting the interest of park managers in tracking rapidly changing landscapes. The Alaska Region is outfitting more planes and training park staff, so response to geohazards can be quicker. Most agency pilots already fly transects for wildlife surveys.

Sources

• National Park Service, Alaska Park Science 20(1), "Mapping and Monitoring Landscape Changes Using Structure from Motion from Aircraft", by Chad Hults, Tahzay Jones, Britta Schroeder, Denny Capps, Dana Hansen, Celia Miller, Michael Hannam and Deb Kurtz, https://www.nps.gov/articles/000/aps-20-1-5.htm; rewritten in hubnx's own words.