Extreme Terrain Overlay

Extreme Terrain marks very steep ground in orange and probable cliffs or abrupt drops in red. These are places where a fall or a slide would be hard to stop, or could carry you over an edge. Steepness Gradient shows the angle at each point, while this layer looks at the shape of the ground around it, which is what it takes to find a drop.

How it works

The model looks at the ground within about 24 m of each point and finds the steepest stretch running through it.

Orange marks very steep ground: that stretch averages 40° or more over at least 6 m of vertical drop, about two stories.

Red marks a probable cliff or rock band. It shows up where the ground averages 55° or more over at least 11 m of drop, or where the terrain has the shape of a rock band.

A rock band has a recognizable shape: a lip that stands out above the slope and a base that sits below it. Looking for that shape lets the model catch smaller bands that the elevation data has smoothed into ordinary steep ground. The base of a cliff usually shows orange, and where both colors apply, red wins.

Zoomed out, only larger walls show up. More detail appears as you zoom in.

About the name

In avalanche education, "Extreme" is also the top class of the Avalanche Terrain Exposure Scale (ATES). ATES rates how exposed a route or area is to avalanches, using slope angle, forest cover, terrain traps, runout zones and more (avalanche.org). This overlay only finds steep ground and drops, so it doesn't rate avalanche exposure, and terrain with no color can still be serious avalanche terrain. Some areas, like Kananaskis Country in Alberta, publish ATES ratings for popular routes, and those are worth checking where they exist.

Built from a digital elevation model (DEM). Cliffs and drops are found using elevation data, so what the overlay can catch depends on the detail available where you are. Where only 30 m data exists, short cliffs and rock steps often fall between data points and won't appear. Not every red area is a real cliff, and a slope with no color can still have one. Check in the field.