4 min read · How Snowmate measures your day

Ask a skier how their day went and you'll usually get a distance - "about 30 kilometres." It's an easy number to reach for, and your phone is happy to supply it. But the one I find myself caring about most, as someone who builds a ski tracking app, is a quieter figure: vertical descent, the total height you drop over all your runs.

It tends to match how the day actually felt - and, better still, your iPhone can measure it accurately without you lifting a finger. Here's what it is and how Snowmate captures it.

What vertical descent means

Vertical descent is simply the sum of every downhill drop in your day, in metres of altitude. Ski from 2,400 m down to 1,900 m and you've banked 500 vertical metres; do it again and you're at 1,000. Keep going and the running total is your descent for the day - the "vert", as people who chase it call it.

Across a whole day your height traces a gentle sawtooth: up on a lift, down on a run, and repeat. Add up only the down parts and you have your vertical descent.

Splitting the day into ups and downs

This is the part that matters for a tracking app. Once it knows your height at every moment, it can tell the ascents from the descents just by watching whether you're going up or down - and that single piece of information is what lets it count your runs, measure the drop of each one, and total your vertical. Get the height right and all of it follows.

Why GPS isn't enough for height

You'd expect the phone's GPS to handle this, and for where you are on the map it's excellent - usually good to a few metres. But it's noticeably weaker at how high you are: altitude can drift by 10–15 metres while you stand perfectly still, which is more than enough to invent runs that never happened. (I went into why in how accurate iPhone GPS is for skiing - the short version is that there's a whole planet blocking the satellites it would need.)

How Snowmate measures it: the altimeter

The fix is a sensor most people don't know they're carrying. Every iPhone since the iPhone 6 has a tiny barometric altimeter - a pressure sensor - and air pressure changes predictably with height. For measuring changes in altitude it's in a different league from GPS: it resolves differences of well under a metre and reacts the instant you drop into a run. So Snowmate lets each sensor do what it's best at - GPS for position, the altimeter for the crisp height changes your descent is built on.

All you do is start and stop

Here's the part you'll never have to think about. You don't mark your runs or tap anything on the lift. You start the tracker at the top of the day and stop it at the end - that's the whole interaction. From the height signal alone, the app sorts out every ascent and descent, counts the runs and adds up the vertical. If you'd like the practical side of keeping it running all day, our guide on how to track your ski & snowboard runs on iPhone covers it.

So that's vertical descent: the quiet number that remembers how much mountain you actually skied - and one your phone is happy to record for you all day, if you let it.