Steepness Overlay
Steepness Gradient colors the map by slope angle, meaning how steep the ground is in degrees. Slope angle is one of the few avalanche factors you can check before you leave home, and this layer shows it for an entire tour at once. Use it to plan where you go, and to spot avalanche terrain you might not have noticed from the trailhead.
Reading the colors
| Band | What it usually means |
|---|---|
|
Light green to green 15–25° |
Low-angle terrain. Easy skinning and mellow skiing. Usually too shallow to start an avalanche. |
|
Yellow 25–30° |
Steeper. Avalanches rarely start here, though wet slides and the occasional large slab can. |
|
Orange to red 30–40° |
Avalanche terrain. This is the range where slab avalanches start most often. |
|
Magenta to blue 40–60° |
Very steep. Slabs still release here. Above about 50°, snow tends to sluff off in small loose avalanches before slabs can build. |
|
Black 60°+ |
Assume it's a cliff or near-vertical rock. |
Slope angle and avalanches
Most avalanche deaths are caused by slab avalanches, where a cohesive layer of snow breaks off a weaker layer below and slides as a block. That takes a slope steep enough to slide, and avalanche educators put the line at 30°. In a Swiss dataset of more than 1,000 avalanches, 96% released on slopes between 30° and 50° (avalanche.org). That's about the steepness of a black or double black run, so the best skiing and the most avalanche-prone terrain are often the same slopes.
Avalanches also run out onto flatter ground, so you're in avalanche terrain if you're on a slope over 30° or underneath one (Explore Big Sky).
Using it
Start with your local avalanche forecast, then use this layer to see where its avalanche problems meet slopes over 30°. Check your whole route, including the skin track, and look uphill for steep slopes above you. In the field, measure slopes with an inclinometer and leave a few degrees of margin. If the ground doesn't match the map, trust the ground.
What it doesn't replace
Steepness Gradient helps with one part of avalanche safety, which is knowing where avalanche terrain is. North American avalanche centers teach five steps through the Know Before You Go program (NWAC):
- Get the gear. Everyone in the group carries a transceiver, shovel and probe and knows how to use them.
- Get the training. A field-based avalanche course teaches you to read terrain, snowpack and weather, and to rescue a partner.
- Get the forecast. Read your local avalanche center's forecast for the day and area you're going.
- Get the picture. Watch for signs of unstable snow, like recent avalanches, cracks shooting out from your skis, or the snowpack collapsing with a whumpf.
- Get out of harm's way. Choose terrain that matches the conditions, and expose one person at a time.
This layer is a planning aid for step 5. None of the others can be done from a map.
How precise it is
Slope angles calculated from elevation data are estimates. With 10 m data they're usually within a couple of degrees, though errors of 3–4° happen, and 30 m data is less precise (Explore Big Sky). A few degrees matter most for slopes near 30°. Short convex rolls and wind drifts can also be a bit steeper than the map shows, so it's worth getting in the habit of checking key slopes with an inclinometer.
Built from a digital elevation model (DEM). Slope is calculated from elevation data, which ranges from about 1 m to 30 m per cell depending on the region. Each colored cell is an average over that cell, so short steep rolls, cliff bands and gully walls can read flatter than they are, especially where only 30 m data exists. Drifts and cornices also change the slope you'd actually ride. Use the colors to plan, and carry an inclinometer to check angles in the field.