Two common kinds of altitude

1) Barometric altitude (pressure‑based)

This is the standard altitude used by air traffic control. The aircraft measures air pressure and converts it to an altitude value. Above a certain height, pilots set a common reference pressure (called standard) so every aircraft is using the same baseline, even when weather pressure changes on the ground.

That’s why you’ll often see consistent cruise altitudes like 33,000 or 35,000 feet.

2) GPS altitude (satellite‑based)

GPS can also provide altitude, but it’s measured differently. Some aircraft/feeds include it, some don’t. GPS altitude may be closer to the aircraft’s “true” position in space, but it isn’t the primary altitude used for separation in controlled airspace.

MSL vs AGL (sea level vs ground)

Altitude can be described two ways:

• MSL (Mean Sea Level): altitude above sea level. This is the common aviation reference.
• AGL (Above Ground Level): height above the terrain directly below the aircraft.

Most public tracking feeds report altitude in a way that matches MSL/flight levels. That means a plane over the Ozarks and a plane over flat farmland could both show 10,000 ft, even though the true height above the ground is different.

What does FL350 mean?

FL stands for Flight Level. At higher altitudes, pilots use flight levels based on standard pressure. FL350 means roughly 35,000 feet on that standard setting. You’ll see flight levels more often during cruise.

Why do planes cruise so high?

Two big reasons: efficiency and weather. Higher altitude air is thinner, so the aircraft experiences less drag and can be more fuel‑efficient. Flying above much of the weather can also make flights smoother.

How vertical speed fits in

FlightsAboveMe also shows whether an aircraft is climbing, descending, or cruising. That comes from vertical speed (how quickly altitude is changing). A value near zero usually means the aircraft is settled at cruise.

Why trackers can disagree

It’s normal to see small differences between flight tracking sites. Common reasons include:

• Different sources (direct ADS‑B vs blended feeds)
• Different smoothing/filters for noisy points
• Latency (one feed is a few seconds behind another)
• Barometric vs GPS altitude reporting

Transition altitude and local pressure

Closer to the ground, aircraft may use the local pressure setting (often called QNH) so altitude lines up with known terrain and airport elevations. At higher altitudes, they switch to standard pressure and talk in flight levels. This “switch point” varies by country and region.

Quick examples

• 3,000–10,000 ft: common for short hops, training flights, and smaller aircraft
• 18,000 ft and up (U.S.): often where flight levels begin
• 30,000–40,000 ft: typical airliner cruise range

Bottom line

For everyday curiosity — “how high is that plane?” — the altitude shown is a solid approximation. But because data can be delayed or filtered, and because “altitude” can be defined multiple ways, FlightsAboveMe (and any tracker) should never be used for navigation or operational decisions.

Quick questions

Is altitude the same as “height above my house”?

Not exactly. Trackers usually show altitude referenced to sea level (or flight level standard), not pure height above the local terrain.

Why does the altitude jump a little?

Live feeds can update in bursts, and different sources may smooth points differently. Small jumps are common.

What’s a typical airliner cruise altitude?

Often somewhere around 30,000–40,000 feet, depending on aircraft type, route, weather, and traffic.

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