What does MSL mean in aviation?
In the context of aviation, MSL stands for Mean Sea Level. It is a common term used to measure elevations and altitudes on the ground and in-flight. Understanding MSL is crucial for pilots, pilots-in-training, and aerodrome operators to ensure the safe and efficient operation of aircraft.
What is Mean Sea Level (MSL)?
Mean Sea Level is the average height of the sea level, representing the height of the open ocean surface. It’s the reference point for the measurement of elevations in aviation, as it helps to define the altitude or height of an aircraft at a given location.
- MSL vs AMSL: There’s sometimes confusion between MSL (Mean Sea Level) and AMSL (Above Mean Sea Level). While the two terms are related, they serve different purposes.
| MSL
| | Characteristics
| —|—
| Mean: The average height of sea level.
| Value: Varies depending on the location and time
| Reference: Not specific to a particular landmass
| Not affected: By tectonic plate movements or crustal uplift
| AMSL
| * | Characteristics:
| —|———-
| Above: Signifies the height above another level.
| Measure: The height above local sea level.
| | Value: Defined by the local terrain relief.
| Reference | Specific to a geographical location
| Varies: Depending on geographic location
Why use MSL in aviation?
The use of Mean Sea Level in aviation solves several issues:
- Enables accurate navigation and communication as MSL is a unique and consistent reference point,
- Simplifies the handling of elevation changes between runways and surrounding terrain through the use of a precise and global reference,
- Promotes harmonization and universality across different countries’ airspaces,
- Essential for precision approach and surveillance systems, such asInstrument Landing Systems (ILS) and Automatic Dependent Surveillance-Broadcast ( ADS-B),
- Facilitates weather forecasting and representation of meteorological data relevant to flight operations.
How does MSL compare to other altitude references?
Table: MSL vs. Other Altitudes
| Altitudes
| | Characteristics
| — |—
| MSL
| * | + Sea level reference
|| | QNH QFE | + Non-standard reference
| |
| * | + Airport-specific reference
| AMSL
| * |- | Local terrain relief is taken into account
MSL serves as a bridge between the global sea levels and the local terrain profile, ensuring a unified referencing system.
Challenges and considerations
- Geoid vs Geodetic datum: Modern GPS and satellite-based surveying methods use the North American Datum of 1983 (NAD-83) or the Global Positioning System (WGS-84) geographical datum, which differ significantly from the Mean Sea Level reference.
- Tidal variations: When calculating MSL, average tidal heights must be acknowledged, as sea levels significantly fluctuate due to environmental factors like tides or storms.
- Terrain variations: MSL remains unaffected by local and regional topography, and therefore may not accurately characterize the true elevation of small areas with significant terrain deformations.
- Geographical location: Calculation of MSL can slightly vary between different locations across the globe due to uneven distribution of sea level measurements by latitude.
Conclusion
