Mapping the last frontier

Alaska is the least densely populated yet largest state in the United States. Well-known for its diverse landscape and cold weather, travelling can be difficult during winter months. In the northern parts of the state, where tundra is vast and winter cli-mate is harsh, use of ice roads are popular and necessary for transporting resources. Because the state has more than 3 million lakes that are larger than 20 acres in size, a clear understanding of the landscape is required to best determine where these roads could safely and sustainably be built. The Bureau of Economic Geology (the Bureau), a research unit at the University of Texas at Austin, set out to map a portion of this wild frontier and provide a better understanding of the local habitat, using the Leica Chiroptera airborne LiDAR system to survey the Alaskan Northern Slope in 2014.

A unique landscape
The Alaskan North Slope micro-topography supports various potential fish habitat water bodies and wetland areas within the arctic tundra environment. Shallow thaw lakes, less than 2 metres deep, in general, are a major component of the tundra landscape of the area, where they compose approximately 20 percent of the total area. They are completely ice-free only a few weeks in a calendar year, so we scheduled our field trip accordingly, beginning in mid-July and ending in early August.

The lakes’ depth, ice growth, and decay determine whether they are suitable habitat for wildlife and aquatic fauna, as well as for industrial development.  Ice accumulation is assumed to be 1.5 to 2 m thick in this area, and liquid water most likely lies below in the central basins of these lakes if the water is deeper than 2 m. Survey findings were particularly important because they would reveal lakes deeper than 2 m, suitable for building ice roads, but with potential fish habitat. Findings were also expected to assist other environmental and hydrological assessments in the area.

“With thousands of lakes – with varying turbidity levels – scattered throughout the survey area and challenging weather conditions that limited the airborne survey activities, this was certainly not an easy task,” said John Andrews, a research scientist, who was responsible for ground truthing and overall logistical support. “With airborne LiDAR surveying, though, we were able to obtain very detailed and precise topographic and bathymetric data in areas where traditional survey methods would not be feasible.”

Flying with double the scanning power
A total of 95 lines were flown to cover the entire survey area, where line numbers increased in the westerly direction. The average flight line was approximately 50 km long. To ensure complete coverage, the flight line spacing was set at 160 to 180 m, where the ground laser swath footprint was calculated to be 280 to 290 m wide. To compensate for the changing ground elevation (30 m in the north, 95 m in the south), atmospheric pressure was monitored during flights to maintain a constant flight altitude and swath above the ground.

The Chiroptera uses two LiDAR scanners to acquire topographic and bathymetric data. Data from the topographic LiDAR (red wavelength) was fired at 300 kHz and used to acquire high-resolution 3-D positional data on vegetation height and earth topography. Data from the bathymetric LiDAR (green wavelength) was emitted at 35 kHz and used to determine water related statistics; such as depth, volume and area size. We also collected colour-infrared and natural-color imagery at 400 m and 1700 m, respectively, for visual reference and ortho-rectification purposes.

“The technological cornerstone of this project was the Chiroptera airborne LiDAR and imaging system,” said John Hupp, a research scientist from the Bureau, who was responsible for field data processing and system calibration. “Simultaneously collecting high resolution imagery with the LiDAR data allowed us to easily discriminate water bodies, vegetation characteristics, wetlands, and uplands, saving us time and costs compared to any other conventional type of surveys.”

For both LiDAR scanners, the average vertical offset was measured at less than 1 cm, while the standard deviation was calculated at approximately 3 cm compared to the ground control points collected at Deadhorse airport runway pavement. Calibra-tion procedures were applied to both scanners individually, where average roll and pitch biases were measured to be less than 2.6 cm.
 
“We also examined and corrected any evident LiDAR system calibration errors caused mostly by incorrect inertial navigation system (INS) rotation angles of roll, pitch, and yaw. These errors can be detected through analysis of adjacent and op-posing LiDAR strips,” said Hupp. “In theory, if no rotational misalignments are pre-sent, LiDAR points registered from different strips should match each other seam-lessly on an unobstructed surface; although not expected to have perfection, we can achieve very close results in practice.”

Faster, more accurate data analysis
Leica LiDAR Survey Suite LLSS v2.09 was used to convert raw data files into indus-try-standard LAS1.2 for output. Because LAS datasets are in binary format, they pro-vide quick and easy access to information, either for analysis or visualization purposes. Datasets from both scanners were tiled to 1 x 1 km to simplify the computational requirements for data viewing and analysis. As a result, we generated 829 tiles across the survey area, and each tile included a 20 m buffer zone in each direction to generate a seamless 1 m digital elevation model (DEM) for mapping purposes.

The deepest water body was calculated at 3.5 m. Of all 4,697 water bodies analysed, 3,837 (81.7 percent) were classified as shallow or very shallow, with measured depths of less than 1 m. Only 4.6 percent (216 total) of the water bodies had depths that exceeded 2.0 m. The average depth of all water bodies was calculated at 0.67 m.

A total of 3,014 water bodies (64.1 percent) contained less than 1,000 m3 of water volume whereas 1,683 lakes were calculated to have more than 1,000 m3 of water volume (35.9 percent). The average volume of all water bodies analysed was calcu-lated at 12,771 m3 (3,373,741 gal) of all water bodies analysed.

“The advanced technology of the Chiroptera provided accurate, detailed, and cost-effective results that permitted analysis of micro-topographic and bathymetric fea-tures in a remote location of the world,” said Andrews. “Water bodies of all shapes and sizes—riverine environments, wetlands and uplands, hills and flat areas, and all other terrain features—were mapped and analyzed rapidly and accurately.”

Written by Kutalmis Saylam