The calibration difficulty index
A single 0–100 difficulty score for every ski run, computed the same way everywhere — so a “blue” at one resort is comparable to a “blue” at another.
Why a calibrated score
Resort difficulty gradings are self-assigned and inconsistent: one mountain’s blue is another’s black. Every run, though, has objectively measurable terrain. We measure it from open data and turn it into one number that means the same thing everywhere.
How it’s computed
Perceived difficulty is driven first by the steepest sustained pitch and second by how sustained that steepness is over the whole run:
score = 0.75 × curve(max gradient) + 0.25 × curve(mean gradient)curve() maps a slope angle onto 0–100 using anchors chosen to line up with how runs are actually skied:
| Slope angle | Score band | Skis like |
|---|---|---|
| 0–16° | 0–40 | Green / easy |
| 16–22° | 40–60 | Blue |
| 22–35° | 60–92 | Black |
| 35°+ | 92–100 | Double black |
What it is and isn’t
The score is an absolute function of measured gradient, not a ranking within the current dataset — so it doesn’t shift as more resorts are added. Gradient comes from OpenStreetMap piste geometry sampled against national elevation models (Geoscience Australia in Australia, LINZ in New Zealand), so the steepest-pitch figure is an estimate at ~8–30 m resolution, not a survey. Conditions (ice, powder, moguls) aren’t in the score — those are what user ratings add on top.