Aircraft altitude is not the distance to the ground
The altitude on the map is measured with a barometer, and above a certain height it is not height above sea level at all. Here is why that is deliberate.
Two aircraft in the same airspace can show different altitudes on the map, even if they are flying at exactly the same level. And the altitude you see is often not the distance down to the ground. Both are entirely normal, and are because aviation measures altitude with air pressure, not GPS.
A barometer, not a ruler
The altimeter in an aircraft is a barometer. Air pressure falls steadily with height, and the instrument converts the pressure into feet. It is an old solution, but it has a decisive advantage: all aircraft that set their altimeter the same way get the same reading at the same level — even if the instruments should be slightly inaccurate. And in aviation, it is precisely the separation between aircraft that matters, not the absolute height above sea level.
What the aircraft broadcasts on ADS-B, and what Kartradar shows, is this barometric altitude.
Why altitude is not the distance to the ground
Air pressure at sea level changes with the weather. In a low-pressure system the pressure is lower, and a barometer that assumes normal pressure will then read too high. Aviation solves this by doing the opposite of correcting everything all the time:
- Low down, pilots set the altimeter to the actual local pressure, reported from the ground. The altimeter then shows height above sea level, which is what you need near mountains and runways.
- High up, above a set transition altitude that varies from area to area, everyone switches to the same standard pressure, 1013.25 hectopascals. The altimeter then no longer shows height above sea level, but a level in a shared, imaginary sea of air. This is what is called a flight level, and it is why "FL350" means the same thing for every aircraft on the route.
If you fly at FL350 on a day with a deep low, you are in reality a few hundred feet lower than 35,000 feet above sea level. It does not matter, as long as everyone around you has the same error.
What GPS altitude would give instead
The aircraft also know their own GPS altitude, which is the geometric distance up from a mathematical model of the Earth's surface. It is unaffected by the weather, and is often tens to hundreds of feet away from the barometric one. On the map that would give an odd result: two aircraft keeping safe separation in the same airspace would show altitudes that did not match the levels air traffic controllers actually work with.
Kartradar therefore shows barometric altitude, the same value used in aviation.
The altitude jumps when the aircraft climbs through the transition
When an aircraft switches from the local pressure setting to standard pressure during a climb, the reading changes abruptly, without the aircraft doing anything special. If you see an altitude jump a few hundred feet in one step on the way up or down, this is usually the reason, not a measurement error.
Speed is measured a third way
The speed in the panel is ground speed: how fast the aircraft moves over the ground. It is not the same as the speed through the air, which is what the pilots fly by. With a 150-knot tailwind, an aircraft can have a ground speed far above what the instruments in the cockpit show, and correspondingly lower the other way. That is why two identical aircraft on opposite headings at the same altitude can show very different speeds on the map.
And the vertical rate
Climb and descent rates are given in feet per minute, as aviation does. A typical airliner climbs 1,500–2,500 feet per minute just after take-off, and descends more gently on its way in to land. Large negative numbers at low altitude are almost always an aircraft on a normal approach, not anything dramatic.
In short
| What you see | What it actually is |
|---|---|
| Altitude | Barometric, measured with air pressure. Above the transition altitude it is a shared level, not height above sea level. |
| Speed | Ground speed, affected by wind. Not what the pilot flies by. |
| Climb/descent | Feet per minute, as aviation measures it. |
| "On the ground" | The aircraft is still transmitting, but without a meaningful altitude. |