The short version
For a person standing on the ground, elevation and altitude are the same number, which is why most people use the words interchangeably and why our altitude page and elevation page show the same ground figure.
- Elevation is the height of a point on the Earth's surface above mean sea level. A mountain, a town and a runway have elevations.
- Altitude is the height of an object or a person above a reference level, usually sea level. A plane, a balloon and a drone have altitudes.
- Height is the vertical size of something, or its distance above the ground directly below it. A building has a height; a drone flying 100 feet above a field is at a height of 100 feet.
The aviation terms
Pilots need precise words because mixing them up can mean flying into a hill. The ones you are most likely to meet:
| Term | Meaning |
|---|---|
| Field elevation | Height of an airport's runway above mean sea level |
| True altitude | Height of the aircraft above mean sea level (MSL) |
| Absolute altitude | Height of the aircraft above the ground below it (AGL) |
| Indicated altitude | What the altimeter shows, set to the local air pressure |
| Pressure altitude | Altitude shown with the altimeter set to the standard 29.92 inches of mercury |
| Density altitude | Pressure altitude corrected for temperature; the altitude the aircraft "feels" |
The surveying terms
Surveyors distinguish between orthometric height, which is height above the geoid (their precise model of mean sea level), and ellipsoidal height, which is height above a smooth mathematical ellipsoid. GPS naturally produces ellipsoidal height, and converting it to orthometric height needs a geoid model. That difference, often around 30 metres in the United States, is the main reason GPS altitude can disagree with a map.
In the United States, official elevations are referenced to the North American Vertical Datum of 1988, known as NAVD 88. The National Geodetic Survey plans to replace it with a new datum based on satellite gravity measurements, which will shift published elevations by around a metre in some areas.
Why "sea level" is not one level
The sea surface is not flat. Tides, currents, winds, temperature and the uneven pull of gravity make it higher in some places than others. Mean sea level is an average taken over many years at tide gauges, and even that differs from coast to coast. The geoid is the scientific version of the idea: the surface sea level would follow if the oceans were at rest, extended under the continents.
That is why elevations are always tied to a named datum. Two maps using different datums can disagree by a metre or more about the same spot, and both be right by their own definition.