It is defined as a plane’s height above the mean sea level, which is a fixed point on the ocean surface.
So, a plane’s MSL is constant regardless of the terrain elevation below it.
Pilots use MSL to calculate a plane’s true altitude during flights and only switch to AGL (Above Ground Level), which is a plane’s height from the ground below, when landing.
Generally, pilots use MSL while flying and AGL while landing.
Pilots use MSL because it lets them measure their altitude above sea level.
The sea level is a predetermined zero point for measuring height, meaning MSL helps them calculate their true altitude.
The mean sea level remains constant regardless of the elevation of the terrain the plane is flying over.
For example, a plane could fly 10,000ft over flat terrain with an elevation of 500ft above mean sea level, so it’d be flying at 10,500ft MSL and 10,000ft above the ground.
Let’s say the plane that flies over a hill with an elevation of 2,000 ft and maintains its altitude.
Now the plane is flying 8,000ft above the ground despite maintaining its altitude.
As a result, the distance from the ground is not a good measurement for altitude.
Instead, the pilot would use their MSL, which remains at 10,500 ft regardless of when the plane flies over the 500 ft plain or 2,000 ft hill.
Differences between MSL and AGL
AGL stands for ‘Above Ground Level,’ and it’s a plane’s height from the ground.
In contrast, MSL stands for ‘Mean Sea Level,’ and it’s a plane’s height from sea level, which is a predetermined zero point.
A plane’s AGL changes when the plane’s altitude or the elevation of the ground below.
In contrast, a plane’s MSL remains constant unless it changes its altitudes.
When Pilots Use AGL instead
Pilots use altimeters, which measure AGL when flying the plane at low altitudes, to land at airports.
The altimeter reading is the most accurate at low altitudes, so it usefully informs the pilot how far their aircraft is from the ground.
Since AGL measures a plane’s height from the ground, it’s more useful than MSL for landing.
Pilots don’t always use AGL for landing because the ground below may be too uneven.
Pilots Fly MSL More Often
Pilots use MSL to express their plane’s altitude, except when landing because AGL is more useful.
Pilots don’t fly with AGL often because it has multiple issues.
Firstly, a plane can’t fly at a constant AGL because the ground below elevation changes, so a plane’s AGL would change even if it didn’t ascend or descend.
For example, a plane flying 500ft AGL above a 2,000ft tall hill would technically be at 2,500 MSL. Even if the plane maintained this altitude, its AGL would decrease after flying over the 2,000ft hill.
Notice that its MSL would stay constant because the mean sea level is a fixed point.
So, the standard civil aviation procedure is to switch to MSL above a particular altitude, known as the transition level.
Therefore, most planes fly with MSL during most of the flight.
Altimeters Measure AGL
Altimeters measure AG, as an altimeter measures the plane’s height above the ground below.
So, a plane’s altimeter reads 0 on the runway and increases as the plane takes off, and the distance between it and the ground increases.
How to Calculate MSL
MSL is calculated using observations of tides and seasonal variations over 19 years to calculate an average MSL.
The average MSL is the zero point from which a plane’s height is calculated.
So, a plane flying at 10,000 ft MSL stays at 10,000 ft MSL regardless of the ground’s elevation.
MSL Measurements & Accuracy
Experts use multiple measurements, including tides and seasonal variations, from different ocean levels to calculate average MSL.
Average MSL changes by less than one-tenth of an inch annually.
Since the earth has an ellipsoidal shape instead of being completely round, the ocean surface is not smooth or consistent.
The result is that different countries and regions define different mean sea levels. For example, the mean sea level defined by China is different from the one defined by Chile.
Generally, MSL calculations are approximated by most countries, and these readings often differ by a few meters.
In conclusion:
MSL, or mean sea level, is a plane’s height above the mean sea level.
Pilots use MSL to calculate their true altitude since the alternative, AGL, uses a plane’s height from below ground.
The problem with AGL is that ground elevations differ drastically, so a plane cannot maintain a steady AGL.
Pilots predominantly use MSL for calculating their altitudes during flight and normally only use AGL when descending to land at airports for that reason.
A plane’s altimeter calculates its AGL, while experts use tide data from the earth’s oceans to calculate an average MSL that varies from location to location.
Helen Krasner holds a PPL(A), with 15 years experience flying fixed-wing aircraft; a PPL(H), with 13 years experience flying helicopters; and a CPL(H), Helicopter Instructor Rating, with 12 years working as a helicopter instructor.
Helen is an accomplished aviation writer with 12 years of experience, having authored several books and published numerous articles while also serving as the Editor of the BWPA (British Women Pilots Association) newsletter, with her excellent work having been recognized with her nomination of the “Aviation Journalist of the Year” award.
Helen has won the “Dawn to Dusk” International Flying Competition, along with the best all-female competitors, three times with her copilot.
