Types of Altitude — Aviation Concept

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.

Local QNH (today's MSL pressure): 1013 hPa — sets where actual MSL really isSubscale set in altimeter: 1013 hPa — what the pilot dialled in the Kollsman window

The five altitudes

Indicated altitude

The pilot turns the knob until the desired pressure level (say, 1005 hPa) appears on a pressure subscale on the face of the instrument. As the knob is turned, the height pointers rotate until, when the procedure is completed with the subscale showing the desired 1005, the altimeter indicates the aircraft's height above this pressure level. — Oxford ATPL, Instrumentation, Ch.5 "The Pressure Altimeter"

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

Pressure altimeters are calibrated to indicate true altitude under ISA conditions. Any deviation from ISA will result in erroneous readings... When temperatures are lower than ISA an aircraft's true altitude will be lower than the altimeter reading. — Oxford ATPL, Meteorology, Ch.9 "Altimetry"

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

If the standard pressure of 1013 hPa is set on the altimeter, the instrument will read what is known as pressure altitude — height in the Standard Atmosphere. This is the altimeter setting used when flying above the transition altitude. — Oxford ATPL, Meteorology, Ch.9 "Altimetry"

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

Density altitude can be defined as the altitude in the standard atmosphere at which the prevailing density would occur, or alternatively, as the altitude in the standard atmosphere corresponding to the prevailing pressure and temperature. It is a convenient parameter in respect of engine performance figures. — Oxford ATPL, Instrumentation, Ch.5 "The Pressure Altimeter"

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:

Aircraft maintains 10 000 ft on the altimeter with QNH correctly set (no pressure error). Only the air temperature changes between scenarios.

Cold day

OAT -30 °C

True alt: 9,000 ft (below indicated)

Density alt: 7,000 ft

ISA

OAT -5 °C

True alt: 10,000 ft (equals indicated)

Density alt: 10,000 ft

Hot day

OAT +15 °C

True alt: 10,800 ft (above indicated)

Density alt: 12,400 ft

Cold air is denser → pressure surfaces sit lower → altimeter over-reads true altitude (the dangerous case in cold mountains). Hot air is less dense → density altitude rises → engine, prop and wing perform as if at the higher density-altitude figure (degraded takeoff and climb).

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.

Oxford treats this under the radio-altimeter section rather than giving it a one-line box definition; the airframe-relative quantity the altimeter approximates is described as:

True Height means the height of the aircraft vertically above the surface immediately below. Used more often in connection with radio or radar altimeters than with pressure altimeters. — Oxford ATPL, Instrumentation, Ch.5 "The Pressure Altimeter"

Quick formula card

A few rules of thumb that exam questions live and die on:

Pressure altitude = Indicated + (1013 − QNH) × 30 ft per hPa (Oxford gives 27 ft / hPa at MSL; the 30 ft figure is the rounded pilot-rule value and is what the JAA/EASA computer uses.)
True altitude ≈ Indicated + 4 × ISA-deviation × (Indicated ÷ 1000) (the 4 ft per °C per 1 000 ft correction from Oxford Met Ch.9)
Density altitude ≈ Pressure altitude + 120 × ISA-deviation (Oxford Instrumentation: "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.