NCALM Exploits Short Flight Windows to Map Rugged Alaska Landscape with Leica TerrainMapper-2

A major landslide triggered by rapid warming in Denali National Park and Preserve forced restrictions at the halfway point of the park road in 2021, significantly reducing access to Alaska’s most popular tourist destination. As plans were made for road repairs and a permanent solution, Park officials called on the National Center for Airborne Laser Mapping (NCALM) to capture LiDAR data and create a first-of-its-kind terrain map of the roadway corridor.

Familiar with Alaska’s historically short flying windows and dramatic elevation changes around Mount Denali, NCALM opted to deploy the Leica TerrainMapper-2 airborne LiDAR sensor for multiple summer 2023 projects in the state. In only their first use of the sensor and its supporting software, the team completed their flight and field operations in nearly half the time they would have anticipated using other airborne systems.

Denali National Park (source: www.nps.gov/dena)

Mapping a Shifting Landscape

Permafrost thaw is causing the landscape to shift beneath the feet of visitors in Denali National Park, home to North America’s highest peak. The park, which often attracts more than half a million tourists annually, has left many disappointed that full access to some of its most popular backcountry sites has been curtailed by the massive Pretty Rocks Landslide at about the halfway point of Denali Park Road.

The road stretches 92 miles into the park and is the only access route for vehicles. Repairing damage to the roadway has been an ongoing challenge for the National Park Service (NPS) since the 1960s because the structure was built on top of what are called rock glaciers. These features are ice cored but covered by a rock blanket. The problem is that as the rock glacier has warmed, it has moved faster, taking a section of the road with it.

Even before permafrost thaw triggered the 2021 landslide acceleration, the Pretty Rocks area had been particularly troublesome. This prompted NPS to contract for construction of a bridge around the site, with work beginning in summer 2023.

Landslide acceleration in the Pretty Rocks area

“They started noticing a number of other sections of the roadway where they are seeing evidence of motion,” said Prof. Craig Glennie, NCALM Director. “NPS became interested in building a high-resolution model of the terrain [in the corridor], which they didn’t have.”

The terrain model will enable Denali officials to fully evaluate their plans to maintain the road in the future. The model will serve as a baseline of 2023 conditions that can be used to measure, track, and predict future road movement as warming and thaw continue. Coupled with ongoing elevation mapping, the terrain model may be key to safely keeping the park open to visitors, Glennie said.

Choosing the Leica TerrainMapper-2

Located at the University of Houston, NCALM is a National Science Foundation-funded institution whose mission is to support academic and government researchers with aerial acquisition of LiDAR and hyperspectral data. NCALM has completed more than 500 airborne mapping projects globally since 2003 and maintains one of the largest LiDAR archives in the world.

NCALM has worked with a variety of commercial aerial LiDAR systems and saw two 2023 Alaska projects as ideal for deploying the TerrainMapper-2 for the first time. The system includes a laser scanner and two nadir 150 MP RGB/NIR optical cameras (although imagery was not a deliverable for the projects).

“It has a longer range [than other LiDAR systems] and provides a much more cost-effective solution for acquiring data in complex topographies because the aircraft doesn’t have to do as much terrain following,” said Glennie.

The system is designed for flexible applications in any environment, capable of massive elevation point capture from high altitudes. The powerful 2 MHz LiDAR sensor delivers 5-cm accuracy with unmatched point density in even distributions over rapidly changing mountainous terrains from altitudes of 300 to 5,500 meters AGL.

The ability to collect points reliably and rapidly from constant altitudes was a primary appeal for the Denali Park Road project, Glennie explained, because the corridor has immense changes in relief, including steep cliffs and deep ravines. Additionally, Alaska’s nearly 24-hour summer sunlight might seem perfect for flight operation, but the reality is different. Heavy clouds, fog, winds, and rain – even in summer – can limit data capture to a few hours per week, making each minute in the air a precious resource.

Saving Time in the Air, On the Ground

A small NCALM team of four people including the pilot traveled to Alaska in July 2023. As in their protocol, the field personnel collected extensive ground control of the roadway corridor and its surroundings. They used the Leica MissionPro software to plan 87 flight lines, each about 30-40 kilometers in length, to cover the corridor and a few kilometers of buffer on either side of the road at a 15 to 20 points per square meter density.

In the air, the pilot relied on the Leica FlightPro application to precisely capture flightlines. As each day’s flying was completed, the team used the Leica HxMap software to process the captured point cloud and review the data quality, standard procedure for NCALM projects where a return visit to re-acquire missing or poor data is not an option. The team spent five days onsite with three days of good flying weather.

“The planning software provided by Leica Geosystems, part of Hexagon with the system, was bar none better than the others we use,” said Glennie. “And the processing package, which allowed the team to process data in the field and get a quick look at it to ensure we had coverage, was two to five times faster than our crew was used to.”

Overall, NCALM completed the 500-square-kilometer project – both on the ground and in the air – with a time savings of 40 percent compared to what would have been possible with other airborne LiDAR systems.