A basic inclinometer—a good tool for measuring slope angle, but be aware of potential inaccuracies when measuring and how we use that information in avalanche terrain.
Ian McCammon presents some compelling data that should make us think twice about basing decisions in avalanche terrain primarily on slope angle. We’re human after all—be slope measurement aware, and be aware of potential measurement inaccuracies.
I’d have to dig through my gear—I’m almost certain somewhere in the depths of my enthusiast “snow science” kit is the original Life-Link inclinometer. (Disclaimer—Just because I have no evidence of said inclinometer doesn’t mean it’s not shoved in the dark recesses of a ski pack.) You might know these simple, analog tools as slope meters. With the BCA version, the slope meter uses a free-rolling ball bearing that slides along a track with marks noting the slope angle. This tool, for example, reads upwards of 60 degrees. Many of use the level app on our phones to measure slope angle.
Slope angle—it seems a rather objective dataset. Maybe not.
According to Ian McCammon in a paper titled Slope Measurement for Humans: Inclinometer Error and Risk Communication, “under ideal conditions, currently available inclinometers have a profile measurement accuracy of about ± 4°. This measurement uncertainty, when combined with start zone avalanche frequency and slab extension effects, can result in significant un-intended exposure to avalanche risk.”
I listened to McCammon present his findings at the 2023 International Snow Safety Workshop (ISSW), and you can catch a video of a similar talk from the 2023 WYSAW (video posted below). In short, McCammon highlights how a basic measurement, like slope angle (slope steepness), can be mismeasured and then misapplied when deciding what slopes to ski and what slopes to avoid.
McCammon uses the premise that we are trained to avoid slopes tipping beyond ~35° when snow instabilities might be present. Thus, slope angle becomes a key factor when determining the terrain we seek to ski/ride. Presumably, if a local avalanche forecast calls for heightened avalanche awareness, generally speaking, we avoid the aspects and slopes where these instabilities are likely to present. This means we avoid those specific aspects and the 35°+ terrain. And if feeling more conservative, one might avoid the 30°+ terrain altogether.
Slope Angle—Getting the Measurement Right
Aside from analog inclinometers, many use apps like Gaia, CalTopo, FatMap, and onX and default to using slope angle shading in those tools. McCammon reminds us that our apps’ slope angle, despite the reliance on high-tech magic and Lidar, can vary by as much as +/- 6 degrees. If we count on sending lines no greater than 35° and wholly rely on a navigation app for slope angle information, know this data could be way off. This means that you could be entering terrain you think is under 35° when, in fact, it is greater.
Take the time to measure slope angle in the field.
McCammon has some compelling data when it comes to measuring slope angle using an assortment of available tools under what he calls “ideal” conditions: “in a warm classroom, with an unambiguous image projected on a large screen and measured with their own inclinometer. I minimized parallax errors by ensuring measurements were made perpendicular to the center of the screen. I also asked participants not to share results until after their measurement forms were handed in.”
Table 1
For our purposes here at THR, let’s zero in on Table 1 (posted above) from McCammon’s paper and focus on the measurement uncertainty, which, cause potential slope angle uncertainties. No matter the tool, the +/- ranges are alarming. For example, there’s a +/- of 4.1° using a ball bearing type inclinometer. This means that a measurement using a ball inclinometer is good within 4.1°. The digital inclinometer I might use on my phone does a smidge better at +/- 3.2°; that’s still a 6.4° window.
Here’s a plug—invest some time watching McCammon’s presentation. He goes into more detail than we will here and it is compelling information.
Slope Angle Aware in Avalanche Terrain
Humans sometimes botch slope angle measurements, or any measurement for that matter. But McCammon reminds us that errors might have greater consequences in higher angle terrain. All this might not be a big deal in 0-20° terrain, especially if no overhead hazards exist. But that scenario changes in “transitional” 20-30° terrain and even more acutely on 30°+ slopes. Imagine a situation where a party member measures the slope angle at 29° with a phone’s level app. According to McMammon’s findings, measuring with the digital app has a +/- 3.2° measurement uncertainty. In this scenario, the slope in question may, in reality, be pushing beyond 32°. Your group should have a firm grasp on how the day’s avalanche problems may manifest on the slope—maybe you would make an altogether less conservative decision on high 20° slopes than you might on slopes trending into the mid 30° range. And noting the measurement uncertainties in Table 1, in steeper 30°+ terrain, a group should gather more information than slope angle alone before committing and dropping in.
In his presentation, McCammon suggests creating a basic triangle with a height of X and a hypotenuse of 2X. The slope should be 30°. Then, practice with your preferred slope angle tool, and note any measurement inaccuracy. If your tool of choice skews, say, +1.5°, then maybe mark the tool with tape, etc., with the measurement error. Again, as humans, there will be some variability, no matter how precise we try to be, in our slope angle measurements—still, measuring slope angle in the field can be trickier than under “ideal” conditions.
Be slope measurement aware, and be aware of potential measurement inaccuracies in avalanche terrain.
