Plane & Pilot
Tuesday, November 1, 2005

How High Are We Now?

We all fly at erroneous altitudes—even when accompanied with a GPS. Here’s how to determine and understand the best way to get the most precise reading.

How High Are We NowIf you have a GPS and a blind encoder in your panel, you may have three independent ways to determine your altitude. But which one is most accurate? We all grew up on baro altitude, so after a short review, we’ll plunge into the GPS world of the WGS84 datum, your height above ellipsoid (HAE) and mean sea level (MSL) altitudes.

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Sea level in Spain is different from sea level in California. I don’t mean the differences caused by the tides—those are averaged out and we call it MSL. The gravitational pull at sea level may be different in the two places because the Earth isn’t made up of uniform stuff. The mass density of material within the Earth’s interior is variable, so gravitational forces vary at a fixed Earth radius.

What that means is that the best definition of the Earth’s MSL is a surface of constant gravitational force that most closely fits all the seas on Earth. That irregular surface is called the Earth’s geoid surface, and the different mean levels of each of the seas on the Earth match that quite well.

Remarkably, even though there’s a 21-kilometer difference in major and minor axes, the maximum difference between the geoid and WGS84 around the globe is about minus-105 meters and 87 meters. In southeast India, the geoid is 105 meters below the ellipsoid, while it swells to 87 meters above it in eastern Indonesia.

You can pull up maps from the Internet showing the Geoid Height (see figure below), which is defined as the difference between the geoid (or MSL) and WGS84. In Los Angeles, the geoid is about 32 meters below the ellipsoid, and the variation throughout the U.S. is between minus-seven meters to about minus-51 meters (MSL surface is below the spheroid).

Your GPS altitude solution gives your HAE, but what’s your MSL altitude? If your GPS navigator can make the geoid height correction for your location, you’d have a pretty accurate measure of your MSL height using the satellite solution. Said differently, if there’s a database of points around the U.S. where the corrections between HAE and MSL are known, the unit can interpolate to estimate that correction where you are and tell you your MSL altitude.


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