What is the difference between MSL and AGL?

MSL is altitude measured from mean sea level. AGL is height above the ground directly beneath the aircraft. AGL changes as terrain rises or falls, even when the aircraft holds one MSL altitude.

Is the altitude shown by my drone controller AGL?

Often it is height relative to the takeoff or home point, not a live measurement of the ground directly below the aircraft. Manufacturer behavior varies. Over changing terrain, home-relative height and true AGL can differ substantially.

Are sectional chart airspace altitudes MSL or AGL?

Most charted airspace floor and ceiling numbers are MSL unless the chart says SFC. Class E transition areas commonly begin at 700 or 1,200 feet AGL and use distinctive vignette boundaries rather than a shelf fraction.

What does 1549 (612) beside an obstruction mean?

The obstruction top is 1,549 feet MSL. Its height is 612 feet AGL. Those values imply local ground near 937 feet MSL.

Can a Part 107 drone fly above 400 feet AGL?

The normal limit is 400 feet AGL. Section 107.51 also contains a structure provision when the aircraft remains within 400 feet of a structure and does not fly more than 400 feet above its immediate uppermost limit. Airspace authorization and every other operating rule still apply.

MSL / AGL / HOME

Part 107 altitude references

Know what the altitude is measured from.

Move one aircraft across changing terrain. The lab keeps the MSL axis fixed while true AGL and controller home-relative height separate in real time.

Open the altitude lab

The equation

AGL = aircraft MSL minus local ground MSL.

Aircraft

2,350 MSL

−

Ground

2,050 MSL

Height

300 AGL

Scaled cross-section

Drag the aircraft. Watch the references split.

Try the ridge and valley first. Keep aircraft MSL steady, move sideways, and watch AGL change with the ground beneath it.

Linear axes with exact readouts

Altitude reference lab

Pick terrain, move the aircraft, then compare four different numbers.

Climb toward a ridge

The ground rises under the aircraft, so home-relative altitude can overstate true AGL.

Drag aircraft or use arrow keys

Both axes are linear. The vertical and horizontal scales are different so altitude changes remain readable.Scale top: 5,000 ft MSL

AGL follows the terrain.

Subtract local ground MSL from aircraft MSL.

MSL stays on one datum.

A fixed MSL shelf does not bend with the terrain.

Home altitude is a third number.

It is not automatically true AGL away from takeoff.

Reference map

Ask what zero means.

Before using any altitude number, identify its zero point. Sea level, local ground, and the takeoff point are three different references.

MSL

One fixed vertical datum

Airport elevation, obstruction top, MEF, most airspace shelf floors and ceilings

AGL

Height above local terrain

Part 107 operating height, obstruction height in parentheses, METAR cloud bases, Class E 700/1,200 floors

HOME

Height relative to takeoff

Common controller telemetry reference. Useful, but not automatically true AGL over changing terrain

Sectional obstruction example

1549 (612)

Top in MSL. Height in AGL.

The top is 1,549 feet MSL. The obstruction rises 612 feet above its local ground. Subtracting the two places the base near 937 feet MSL.

Open the full sectional symbol key

Frequently asked questions

What is the difference between MSL and AGL?: MSL is altitude measured from mean sea level. AGL is height above the ground directly beneath the aircraft. AGL changes as terrain rises or falls, even when the aircraft holds one MSL altitude.
Is the altitude shown by my drone controller AGL?: Often it is height relative to the takeoff or home point, not a live measurement of the ground directly below the aircraft. Manufacturer behavior varies. Over changing terrain, home-relative height and true AGL can differ substantially.
Are sectional chart airspace altitudes MSL or AGL?: Most charted airspace floor and ceiling numbers are MSL unless the chart says SFC. Class E transition areas commonly begin at 700 or 1,200 feet AGL and use distinctive vignette boundaries rather than a shelf fraction.
What does 1549 (612) beside an obstruction mean?: The obstruction top is 1,549 feet MSL. Its height is 612 feet AGL. Those values imply local ground near 937 feet MSL.
Can a Part 107 drone fly above 400 feet AGL?: The normal limit is 400 feet AGL. Section 107.51 also contains a structure provision when the aircraft remains within 400 feet of a structure and does not fly more than 400 feet above its immediate uppermost limit. Airspace authorization and every other operating rule still apply.