Programming

Flight instructor here: There are several definitions of altitude that pilots are taught about and must consider during a flight. These include:

True altitude: The aircraft's actual height above mean sea level. Nowadays you'd probably get this from a GPS receiver. Most of the time we don't really care about this.

Absolute altitude: The aircraft's instantaneous altitude above the surface directly below it. This might be measured by a radar altimeter if the aircraft is carrying one, or possibly calculated comparing the aircraft's GPS position and a topographical map, a lot of moving map systems can do that these days. Fairly important to keep in mind for avoiding controlled flight into terrain.

Indicated altitude: The reading of a barometric altimeter set to the local barometric pressure. Below 18,000 feet, this is used for vertical separation of aircraft. Air Traffic Control tells you to climb and maintain 8,500, you climb until the altimeter points to that number. This is still above mean sea level so when flying over land your absolute altitude is almost certainly less than your indicated altitude.

Pressure altitude: The reading of a barometric altimeter set to 29.92 inches of mercury. Above 18,000 feet, used for vertical separation of aircraft. 18,000 feet is the floor of Class A airspace: en route IFR-only airspace, it's where airliners cruise. They're not worried about where they are in relation to the surface so much, they're mostly concerned with avoiding other aircraft, and they don't want to have to constantly adjust for local settings, so they sit it to match a standard day. This might mean they're hundreds of feet off from their true altitude, but who cares? This is part of the reason we refer to altitudes up there as "flight levels". 22,000 feet is called Flight Level 220.

Density altitude: This one is going to bend your brain a little. Density altitude is indicated altitude corrected for non-standard temperature. It is used for predicting aircraft performance. Aircraft work by interacting with air molecules. When the air is dense, the engine can generate more power, the propeller can generate more thrust, and the wings can generate more lift. When the air is less dense, you get less thrust and lift. Doesn't matter why the air is less dense; air decreases in density as you climb, and the air becomes less dense as you heat it. It makes sense in a pilot's head to think of this in terms of altitude on a standard day. The most average weather is 15 degrees C and 29.92 in. Hg, so we compare all weather to those conditions and calculate aircraft performance based on those conditions. On a very hot day, we'll do a calculation which tells us what altitude it feels like we're at on a standard day, and that makes intuitive sense to pilots. "Normally this runway is at 1,000 feet, but today it feels like it's at 3,000 feet." Takeoff and landing performance, ability to climb, and true airspeed are calculated from density altitude.