You're cruising at "10 000 ft". The altimeter says so, ATC says so, the chart says you're 7 500 ft above the ridge. But on a hot day at a non-standard QNH, your true height above sea level isn't 10 000 ft, the engine thinks it's at a different altitude again, and the radio altimeter disagrees with all of them.
There isn't one altitude — there are five. Each one is the answer to a different question, measured from a different datum.
The five altitudes
Indicated altitude
First identify the pressure setting in use. The same sensed pressure produces a different displayed altitude when that setting changes. Record both the number on the dial and the setting before comparing two altitude reports.
In plain terms: indicated altitude is just what the altimeter reads with whatever pressure you've set in the subscale. With QNH set, it reads vertical distance above mean sea level (in ISA conditions). With QFE set, it reads height above the aerodrome. With 1013 hPa (the standard pressure setting), it reads pressure altitude — see below.
True altitude
Imagine a vertical measuring line from the aircraft to mean sea level. Its length is true altitude. A pressure instrument estimates that geometry through an atmospheric model; a cold air column can make that estimate too high even when the local pressure setting is correct.
In plain terms: true altitude is your actual vertical distance above mean sea level. The altimeter only shows true altitude when both QNH and the temperature column match the ISA. In colder-than-standard air the pressure surfaces compress, so you sit lower than the altimeter claims (HI to LO, look out below). In warmer air the opposite — true altitude exceeds indicated.
Pressure altitude
To obtain pressure altitude, map the measured atmospheric pressure to the matching level of the standard atmosphere. Selecting 1013.25 hPa provides that reference on a conventional altimeter. It is an atmospheric reference, not a measurement of terrain clearance.
In plain terms: pressure altitude is your vertical distance above the 1013 hPa pressure surface, regardless of where mean sea level actually sits today. Set 1013 in the subscale and you're reading it directly. It's the basis for flight levels (FL100 = 10 000 ft on 1013) and the input to almost every performance calculation.
Density altitude
Instead of matching pressure, match the local air density to the standard atmosphere. The matching level is density altitude. This allows two places with different elevations and temperatures to have comparable density conditions for performance planning.
In plain terms: density altitude is the altitude the engine and the wing think they're at. Hot day → thin air → density altitude higher than pressure altitude → less thrust, less lift, longer take-off run. On a cold day at the same airfield, density altitude can be below field elevation.
The widget below isolates the temperature effect — same indicated altitude in all three scenarios, only the air temperature changes:
Absolute altitude
In plain terms: absolute altitude is your true height above the ground (or terrain) directly beneath you. It's what a radio / radar altimeter reads, and it's what matters for terrain clearance. Above flat sea, absolute altitude equals true altitude. Over a 5 000 ft ridge while at 10 000 ft true, your absolute altitude is 5 000 ft.
The airframe-relative quantity the altimeter approximates is described as:
For a geometric check, subtract the elevation of the terrain immediately below from true altitude. A radio altimeter measures a related height using a reflected signal; its installation, range and the terrain affect the actual indication.
Quick formula card
A few rules of thumb that exam questions live and die on:
- Pressure altitude = Indicated + (1013 − QNH) × 30 ft per hPa (the precise figure is 27 ft / hPa at MSL; 30 ft is the rounded pilot-rule value used by the JAA/EASA computer.)
- True altitude ≈ Indicated + 4 × ISA-deviation × (Indicated ÷ 1000) (the 4 ft per °C per 1 000 ft temperature correction)
- Density altitude ≈ Pressure altitude + 120 × ISA-deviation ("density altitude = pressure altitude ± ISA deviation × 120")
- Absolute altitude = True altitude − terrain elevation directly below
- ISA temp at altitude = 15 − 2 × (alt ÷ 1000) °C, so ISA-deviation = OAT − ISA-temp
Worked example
You're cruising on QNH 993 hPa with the altimeter showing 8 000 ft. OAT at altitude is −15 °C. The terrain directly below peaks at 2 000 ft elevation.
Pressure altitude:
PA = 8 000 + (1013 − 993) × 30
= 8 000 + 20 × 30
= 8 000 + 600
= 8 600 ft
ISA deviation at 8 000 ft:
ISA temp = 15 − 2 × 8 = −1 °C
ISA-dev = OAT − ISA = −15 − (−1) = −14 °C (colder than standard)
True altitude:
TA ≈ 8 000 + 4 × (−14) × (8 000 ÷ 1000)
= 8 000 + 4 × (−14) × 8
= 8 000 − 448
= 7 552 ft
You're 448 ft lower than the altimeter reads — the cold-air "look out below" case.
Density altitude:
DA ≈ PA + 120 × ISA-dev
= 8 600 + 120 × (−14)
= 8 600 − 1 680
= 6 920 ft
Cold air is dense — the engine performs as if it were below 7 000 ft.
Absolute altitude:
AA = TA − terrain = 7 552 − 2 000 = 5 552 ft
Your radio altimeter, if it reached that high, would read about 5 550 ft over the peak.
Common mistakes
- Confusing the QNH→1013 sign. When QNH is lower than 1013, the 1013 surface sits below sea level, so pressure altitude is higher than indicated. A lot of candidates flip the sign and end up with PA below indicated on a low-pressure day.
- Applying the temperature correction the wrong way. "Cold = LO" means true altitude is lower than indicated when it's colder than ISA. The correction subtracts on a cold day, adds on a hot day.
- Mixing up density and pressure altitude. Pressure altitude only needs the QNH; density altitude additionally needs the temperature. On an ISA day at any QNH, density altitude = pressure altitude.
- Equating absolute and true altitude. They only match over the sea (terrain elevation = 0). Over mountains the gap is the terrain elevation, and that's the gap that matters for CFIT.
Why it matters
Three of these altitudes show up in the same ATPL exam question constantly: questions give you indicated altitude, QNH, and OAT, then ask for pressure, density, or true altitude. Get the formula sequence straight and they're worth easy marks. Operationally, density altitude governs every take-off and climb performance chart you'll ever read, and the cold-temperature error correction on true altitude is a real-world terrain-clearance rule — published in the AIP for any aerodrome where winter approaches go below ISA −15 °C.
Further reading
FAA Pilot’s Handbook of Aeronautical Knowledge, flight instruments and FAA AIM, altimeter setting procedures. These links support the underlying concepts; the explanations and examples above are AvioQuiz study material.