Two altitudes, and they disagree
Almost every aircraft on a tracker reports two different altitudes, and they are rarely the same number. Neither is wrong. They are measuring different things, and understanding which is which explains a great deal that otherwise looks like a bug.
| Barometric altitude | Geometric altitude | |
|---|---|---|
| Field | alt_baro | alt_geom |
| Measured by | Air pressure | Satellite navigation |
| Referenced to | A pressure setting | The WGS84 ellipsoid |
| Used for | Separating aircraft | Terrain awareness, navigation |
| Shown by most trackers | Yes, by default | Sometimes, as a second figure |
The one you normally see is barometric, because that is what aviation runs on.
Why aviation still measures height with pressure
It sounds primitive to determine altitude from air pressure when every aircraft carries satellite navigation. It is deliberate, and the reason is that air traffic control does not actually care how high an aircraft is above the ground. It cares that two aircraft are not in the same place.
If every aircraft in a region sets its altimeter to the same pressure reference, then all of them are measuring from the same imaginary surface. Whether that surface is exactly at sea level is irrelevant — what matters is that an aircraft reporting 35,000 and one reporting 36,000 are genuinely 1,000 feet apart. A shared, consistent error is harmless. Inconsistency is not.
QNH, standard pressure and the transition altitude
Below a certain height, crews set the local pressure — the QNH — passed to them by ATC, so the altimeter reads height above sea level. Above that height, everyone switches to the standard setting of 1013.25 hPa, and readings become flight levels: FL350 rather than 35,000 feet.
The changeover height is the transition altitude. In the UK it varies by area and is commonly between 3,000 and 6,000 feet; much of continental Europe uses higher figures. Above it, nobody is measuring true height above the sea at all — they are all measuring from the same convenient fiction.
This is why an aircraft's indicated altitude can shift by a few hundred feet the moment it changes pressure setting, without the aircraft actually climbing or descending. On a low-pressure day the difference is larger. Nothing has moved; the reference has.
Why a parked aircraft says minus fifty feet
This is the single most common confusion, and it follows directly from the above. Trackers usually display the raw pressure altitude the transponder sends, which is referenced to standard pressure whatever the aircraft is actually doing.
On a day when local pressure is higher than 1013.25 hPa, an aircraft sitting on the apron will report a negative altitude. On a low-pressure day the same aircraft reports several hundred feet. The aeroplane has not moved and the transponder is not faulty — you are seeing an uncorrected pressure reading, and the weather changed.
As a rough rule, one hectopascal is about 27 feet. A parked aircraft reading −80 feet is telling you the local pressure is around 1016 hPa.
Why the geometric altitude is not "the real one"
It is tempting to treat the satellite-derived figure as the truth. It is more literal, but it is not height above the ground either.
Satellite navigation calculates position relative to the WGS84 ellipsoid, a smooth mathematical approximation of the Earth's shape. Actual mean sea level — the geoid — departs from that ellipsoid by tens of metres depending on where you are. Over the UK the difference is roughly 45 to 50 metres, so a geometric altitude can sit around 150 feet away from height above sea level before anything else is considered.
Aircraft that report both usually also report the difference between them, which is how a decoder can reconcile the two.
What the vertical rate is telling you
Rate of climb or descent comes in two forms, matching the two altitudes: a barometric rate and a geometric one. Trackers generally show whichever is available, in feet per minute.
| Rate | Typically |
|---|---|
| +2,000 to +3,000 fpm | Initial climb after departure |
| +500 to +1,500 fpm | Climbing in the cruise, or a step climb |
| 0 fpm | Level. Small oscillations are normal. |
| −1,000 to −2,000 fpm | Normal descent |
| −3,000 fpm or steeper | An expedited descent. Usually a routine ATC instruction. |
A steep descent rate is one of the most misread values on a tracker. Rates beyond −3,000 fpm are unremarkable — they happen every day when a controller needs an aircraft down quickly for sequencing.
The selected altitude, if it is transmitted
Some aircraft also broadcast the altitude dialled into the autopilot. It is genuinely interesting, because it tells you where the aircraft is going rather than where it is. An aircraft at FL180 with a selected altitude of FL360 is climbing and has a long way to go; the same aircraft with 7,000 selected is being brought down for an approach.
The altitude you see is a pressure reading against a standard reference, not a height above the ground. It is correct for its purpose — keeping aircraft apart — and slightly wrong for the purpose most people assume. Negative values on the ground are normal, and steep descent rates are usually just a controller getting an aircraft down.