MSL, AGL, WGS84: three altitudes you'll see at 35 000 feet

· Kim Engels

If you’ve ever stared at the altitude on your phone, looked up at the seat-back screen, and found a 200-meter discrepancy, you’re not crazy. Pilots, GPS receivers and map apps speak slightly different altitude languages. Skyty exposes all three, partly because they’re useful and partly because we couldn’t decide which one was the right default. (Spoiler: it’s MSL. We argued for a week.)

MSL — Mean Sea Level

What it sounds like: your height above the average level of the ocean. This is the number most travelers want. “We’re cruising at 11 km MSL.” It matches the altimeter callouts you hear from the cabin crew.

Skyty’s default mode is MSL. Internally, your raw GPS altitude (which is referenced to a mathematical ellipsoid, see below) is corrected by the EGM2008 geoid — a model of Earth’s gravity that says, roughly, “sea level here is X meters below the ellipsoid.” Apply that correction and you get MSL.

AGL — Above Ground Level

Your height above the terrain directly under you, not the sea. Over the Mariana Trench your AGL is much greater than your MSL. Over Mount Everest your AGL would be negative (good thing planes don’t fly there).

AGL is a pilot’s number — it tells you how much air you have between you and the ground. For passengers, it’s also the most viscerally interesting one. When the AGL display drops from 9 000 m to 6 000 m as you cross the Alps, that’s the visual cue to look out the window.

To compute AGL offline, Skyty needs an elevation map of every patch of ground on Earth. We use NASA SRTM terrain data — the same dataset commercial cockpits use. The full pack is ~900 MB and downloads once at home. After that, every GPS fix gets enriched with a local elevation lookup.

WGS84 ellipsoidal — the raw GPS altitude

This is the altitude that GPS receivers natively output. WGS84 is a mathematical ellipsoid that approximates Earth’s shape — flat at the poles, bulged at the equator. It’s close to sea level, but not exactly. Depending on where you are on the planet, ellipsoidal altitude can differ from MSL by anywhere between −105 m (Indian Ocean) and +85 m (North Atlantic).

For most travelers, WGS84 is overkill. For pilots, surveyors and developers debugging GPS code, it’s the source of truth. Skyty exposes it as an option for the engineers in the audience.

Why the seat-back display disagrees with everything

The number on the in-flight entertainment screen is usually flight level (FL) — pressure altitude calibrated to a standard atmosphere of 1013.25 hPa. It’s what air-traffic control uses to keep aircraft separated, and it’s a barometric number, not a geometric one.

On a warm day with low pressure, your true geometric altitude can be 100–300 m higher than the flight level shown on the display. On a cold day with high pressure, lower. So if your phone says 11 200 m and the screen says FL360 (= 10 973 m), neither is wrong. They’re measuring different things.

Which one should you use?

Skyty’s settings let you switch between MSL and WGS84 with one tap; AGL appears alongside whichever you choose, when terrain data is loaded.