Mean Sea Level (MSL)

Mean Sea Level (MSL) is the fundamental reference level used in height calculations, based on the long-term average of tidal measurements of the Earth’s ocean surface. This concept defines a fixed zero point derived from the average of observed rises and falls of the sea surface over a specific time period. It serves as a standard vertical reference in geographic, meteorological, and geophysical measurements as well as in aviation.

Use in Aviation

In aviation, MSL is the primary reference for defining an aircraft’s height above sea level. Pilots and air traffic controllers express altitude data relative to MSL to ensure flight safety. Aircraft altimeters calculate height by measuring differences in barometric pressure, with these calculations based on the standard atmospheric pressure at sea level.

When the altimeter’s pressure setting is adjusted to MSL, the displayed value represents the true altitude—the vertical position of the aircraft measured from sea level. Flight levels, terrain elevations, and obstacle data are also referenced to this same standard. This enables a common altitude language to be used among pilots and controllers across different regions.

Relationship Between Types of Altitude

Altitude measured relative to MSL forms the basis for other altitude concepts. Absolute altitude (AGL) expresses the vertical distance between an aircraft and the ground surface, while true altitude (MSL) defines this distance relative to sea level. Pressure altitude is obtained when the altimeter is set to the standard atmospheric pressure of 1013.2 hPa, and flight levels are determined based on this value. Density altitude is pressure altitude corrected for temperature and is used in aircraft performance calculations.

Practical Significance

The use of MSL as a reference is a critical standard that directly impacts flight safety. Terrain elevations, obstacle heights, and airport elevations shown on maps are calculated relative to this level. This allows pilots to rely on consistent data during route planning and approach calculations. Additionally, MSL serves as the basis for determining vertical separation distances between aircraft in air traffic control systems.

Technical Foundations

Altimeter systems determine altitude by detecting changes in atmospheric pressure. The standard pressure value accepted as reference at sea level is 1013.2 hPa. This value is based on the International Standard Atmosphere (ISA) model, which assumes a sea level temperature of 15 °C and a density of 1.225 kg/m³. The QNH value entered into the altimeter represents the pressure reduced to sea level and enables the altimeter to display altitude relative to MSL.

MSL is a universal reference system at the core of vertical measurement and altitude concepts in aviation. Defined by the long-term average of sea level, it establishes a global standard despite geographic variations. This ensures consistency and safety in flight operations, navigation planning, and air traffic control services.