Assessing and Managing Slope Angle
In discussions after the accident, the group described trying to avoid avalanche terrain. One of the ways they were doing this was by traveling on slopes with an angle less than 30 degrees. Looking at slope steepness was part of their trip planning process and they used common digital tools to look at shaded slope angle layers draped over terrain models or topographic maps. With the help of these tools, they avoided avalanche terrain for most of the day. They also identified the narrow terrain feature where they could keep their slope angle at or below 29 degrees as they worked through much steeper terrain. Their GPS tracks shows that they were either on or very close to their planned route.
We measured slope angles during our visit to the accident site. Measured angles ranged from 32 to 34 degrees on the slope the group was descending when the avalanche released. This is only a few degrees steeper than the shading tools show, but in this case it may have been a very important difference. The group did not attempt to measure the slope angle before their descent. If they had, it is possible that they would have found this discrepancy. On some days and with some avalanche problems, the difference between 29 degrees and 32 degrees might not be significant. However, it can be significant when dealing with a Persistent Slab avalanche problem, especially when a remote trigger is possible.
The difference between a representation of the terrain and the actual terrain is a very important issue for everyone using these tools for route planning. The same issues hold for everything from paper maps to the most sophisticated digital tools. These are very useful tools, but they have limitations and they are most useful when coupled with observations of the physical terrain. In a specific example, the popular mapping software CalTopo derives the slope angle coverage from the USGS National Elevation Dataset (NED). The cells displayed by the software are nominally 10 meters across, though in some locations the underlying source data may be much coarser (Jacobs 2019). The NED could have errors of up to 100 feet in elevation (Scott 2009). Haneberg (2006) found that slope angles for a single cell within a 10 m grid often had a degree or two of error, with typical errors of plus or minus 4 degrees. Most of these tools use the elevation from multiple surrounding grid cells (often 8) to calculate the slope angle (ERSI, retrieved 2019). Thus, elevation errors in one grid cell can affect surrounding cells. With 10 meter cells and a 31-degree slope, a single cell with an elevation 100 feet too high gives a slope angle of 57 degrees, while a single cell 100 feet too low produces a slope angle of 24 degrees. McCollister and Birkeland (2008) examined this issue for avalanche applications and remarked that we are “looking at a depiction of reality. We tend to get excited about the graphics and colors dancing around on the computer screen in front of us and lose track of how that really reflects what is found on the ground.”
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