MSL vs AGL: The Altitude Mistake Pilots Can’t Afford

Altitude sounds simple until the question asks: altitude above what?

That is where MSL and AGL become easy to confuse. For ATPL students, this is a classic exam-pressure trap. You may know the theory, but one quick misread can change the meaning of a question completely. Are you dealing with aerodrome elevation, or cloud base, or minimum altitude, or terrain clearance? The answer depends on the reference. 

In this guide, we will break down the difference between MSL and AGL, when each one is used, how they appear in aviation weather and charts, and what students often get wrong in ATPL-style questions.

MSL Overview

MSL stands for Mean Sea Level.

In aviation, MSL describes altitude measured from mean sea level as the reference point. It answers one simple question: how high is something above sea level? This is also a measure of true altitude: the actual vertical distance above mean sea level.

This does not mean the aircraft has to be flying over the sea. Mean sea level is used as a common reference, so pilots, charts and procedures can describe altitude in a consistent way.

For example, if an aircraft is flying at 3,000 ft MSL, it means the aircraft is 3,000 feet above mean sea level. The terrain below may be flat, mountainous, or anything in between. The MSL value does not tell you directly how much clearance you have above the ground.

Note: true altitude and indicated altitude are not always exactly the same. An altimeter set to QNH gives an altitude referenced to mean sea level, but altimeter errors, temperature and pressure conditions can still affect accuracy. That is where AGL comes in.

AGL Overview

AGL stands for Above Ground Level.

AGL describes height measured from the ground or surface directly below. It answers a different question: how high is something above the terrain? This is also known as absolute altitude: the actual height of the aircraft above the surface below.

For example, if an aircraft is flying 1,000 ft AGL, it means it is 1,000 feet above the ground below it. Unlike MSL, AGL can change quickly even if the aircraft holds the same altitude. If the terrain rises, the aircraft’s height above ground decreases. If the terrain falls away, the aircraft’s height above ground increases.

This is why AGL is closely linked with terrain awareness. It tells you how much vertical space exists between the aircraft and the surface below.

Altitude Basics: 5 Types of Altitude Explained. Review the key altitude types pilots need to understand, including indicated, true, pressure, density and absolute altitude.

Simple Difference Between MSL and AGL

The difference is all about the reference point.

MSL / true altitude measures altitude from mean sea level. AGL / absolute altitude measures height from the ground below.

A simple example makes this clear:

Aircraft altitude: 3,000 ft MSL Terrain elevation below: 1,000 ft MSL

The aircraft is therefore: 3,000 − 1,000 = 2,000 ft AGL. So the same aircraft can be at 3,000 ft MSL and 2,000 ft AGL at the same time.

MSL in Pilot Operations

Pilots use MSL when working with charted altitudes, terrain data and vertical separation. This includes:

• aerodrome elevation, 

• terrain elevation, 

• minimum safe altitudes, 

• controlled airspace limits, 

• cruising altitudes,

• instrument procedure altitudes.

On most aviation charts, altitudes and elevations are referenced to mean sea level unless another reference is clearly stated. This allows pilots to compare the aircraft’s altitude with terrain, obstacles, airspace and published procedures using the same vertical datum.

AGL in Pilot Operations

AGL is the reference pilots use when thinking about real clearance from terrain, cloud base, circuit height, radio height and obstacles close to the aircraft.

This is common in low-level phases of flight: take-off, approach, landing, circuit flying and terrain or obstacle clearance. 

Obstacle information can involve both references, which is where students often get caught. An obstacle may have an elevation in MSL and a height in AGL. Elevation tells you how high the top is above sea level. Height tells you how tall it is above the surrounding ground.

See and Be Seen: Rules for Safe VFR Flying. Review the visibility, cloud clearance and terrain awareness rules that matter in VFR flight.

Are METARs and TAFs AGL or MSL?

In METARs and TAFs, cloud heights are normally reported above aerodrome level, not above mean sea level. In practical terms, this means they are treated as AGL/AAL values: height above the reporting aerodrome.

For example, if a METAR reports: BKN020. This means a broken cloud layer with a base at about 2,000 ft above the aerodrome, not 2,000 ft above sea level. If the aerodrome elevation is 500 ft MSL, then the cloud base is approximately: 500 ft + 2,000 ft = 2,500 ft MSL

This distinction matters because pilots often need to compare cloud base with terrain, minimum altitudes and approach requirements. The same logic applies to TAFs. A TAF forecasts cloud height above the aerodrome, so a forecast such as OVC008 means an overcast layer at about 800 ft above aerodrome level.

This is also where the idea of ceiling becomes important. In aviation weather, ceiling normally refers to the lowest cloud layer reported as broken or overcast, or vertical visibility when the sky is obscured. A scattered layer has a cloud base, but it is not usually treated as a ceiling.

ATPL Exam reminder:

• METAR/TAF cloud height = above aerodrome level.

• Charted altitude/elevation = usually MSL.

• To compare them properly, check the reference first.

Master the METAR: Your Key to Aviation Weather Code. Learn how cloud height, visibility, wind and pressure appear in real aviation weather reports.

Cockpit References for MSL and AGL

The main cockpit reference for MSL is the barometric altimeter. 

When the correct QNH is set, the altimeter indicates altitude referenced to mean sea level. On the ground, it should read approximately the aerodrome elevation. In flight, this allows the pilot to compare aircraft altitude with charted altitudes, terrain elevations, minimum altitudes and airspace limits.

When standard pressure is set, the altimeter no longer shows altitude above mean sea level in the same practical sense. It shows pressure altitude, used for flight levels. The standard setting is 1013.25 hPa or 29.92 inHg.

The main cockpit reference for AGL is the radio altimeter. It measures height above the terrain directly below the aircraft. This is especially useful close to the ground, during approach, landing and some low-level operations.

On a modern PFD, pilots may see both references close together: barometric altitude on the altitude tape, and radio altitude displayed near the lower part of the attitude indicator when the aircraft is close to the ground.

The important point is that these numbers do not come from the same source.

• Barometric altitude depends on pressure setting.

• Radio altitude measures height above the surface below.

For ATPL students, the common trap is assuming every altitude shown in the cockpit uses the same reference. It does not. Always ask: is this barometric altitude, pressure altitude, or radio height?

Six-Pack Basics: Your Guide to Primary Flight Instruments. See how traditional cockpit instruments display the flight information pilots rely on.

Common ATPL mistakes about MSL and AGL

MSL and AGL questions are usually not difficult because of the maths. They are difficult because of the reference. In the exam, always ask: what is this value measured from?

One common trap is obstacle information on charts. On VFR charts, obstacle elevations are normally shown in feet above MSL. This matches the way terrain elevation is shown. So if a chart gives an obstacle value, do not automatically treat it as height above ground.

The useful distinction is:

• Obstacle elevation = height of the obstacle top above MSL.

• Obstacle height = height of the obstacle above the ground.

Another common trap is pressure setting terminology. QNH is based on pressure reduced to mean sea level. With QNH set, the altimeter should show the aerodrome elevation when the aircraft is on the ground.

QFE, by contrast, is referenced to the aerodrome. With QFE set on the ground, the altimeter should read zero, or close to zero. This is why a question comparing QFE, QNH and QFF is really testing aerodrome elevation. If all three values are the same, the aerodrome must be at mean sea level. There is no height difference to correct for.

Students also get caught when moving between weather reports, charts and cockpit instruments. Cloud heights in METARs and TAFs are normally above aerodrome level. Charted altitudes and terrain elevations are usually MSL. Radio altitude is height above the surface below.

MSL, AGL, QNH, QFE, radio altitude and chart elevation all describe vertical position, but not from the same starting point.

Quick exam refresher: MSL vs AGL

Before answering any altitude question, ask one thing first: Measured from sea level, or measured from the ground?

Check Yourself

What does MSL mean in aviation? MSL means Mean Sea Level. It describes altitude measured above mean sea level.

What does AGL mean in aviation? AGL means Above Ground Level. It describes height above the ground or surface below.

Is MSL the same as true altitude? Yes. True altitude is the actual vertical distance above mean sea level.

Is AGL the same as absolute altitude? Yes. Absolute altitude is the actual height above the ground below.

Are METAR cloud heights AGL or MSL? METAR cloud heights are normally reported above aerodrome level, so they are treated as AGL/AAL values.

Next Step

Want to practise altitude-reference questions before exam day? Open the Airhead ATPL Question Bank and test how well you can spot the difference between MSL, AGL, QNH, QFE and radio height.