Which to use: "altitude" or "elevation" in regards to height above sea level?

I've developed an Android application that is used as a Car Home replacement. One of the pieces of information that I return to the user is what I originally called "altitude": the height above sea level the user is currently at while driving.

However, a user sent me an email regarding my use of the word altitude and recommended that I switch it to elevation saying "Altitude is a measurement used to show how far off the ground you are, elevation is used to show how high above sea level you are." I started to believe that what he told me was correct, but after a search I found this interesting comparison of the two. Additionally, Android's API declares this value as "altitude" when returning it from the GPS receiver. Now I'm just confused.

Which word should I be using to express this?

Both altitude and elevation are measures of the height of a point relative to some datum. The differences are in how they are derived and what they are normally used for.

Altitude is typically only used to describe the height of an aircraft in flight. It is a barometric measurement expressed relative to the height of a runway or mean sea level in a given location or region (taking into account current local atmospheric conditions), or to an arbitrary standard datum (to eliminate the effect of localised variations in air pressure).

Elevation is usually used to describe the height of the ground, or a feature fixed to the ground. It is a geometric measurement expressed relative to the mean sea level datum established for the region by the national mapping agency.

I would take some care to understand the meaning of a value named altitude obtained from a GPS receiver. At a basic level, GPS can only tell you the receiver’s height above the WGS‑84 ellipsoid, and it cannot be assumed that the surface of the ellipsoid is the same as mean sea level in any given location. Some receivers contain look-up tables to calculate the offset between the two, given a lat/long position, which ought to be incorporated in the determination of ‘altitude’.

Whether to use altitude or elevation for your application is largely a matter of preference, I’d say. Since a moving car is neither an aircraft in flight nor fixed to the ground, I’ve conveniently excluded it from my definitions of the two terms here. As your measurement is geometric rather than barometric, I’d lean towards elevation. Alternatively, you could just keep the terminology simple (if imprecise) and call it height…

The problem with using altitude is that it is a relative measure with a non-standard reference point. Or as wikipedia puts it:

As a general definition, altitude is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object

Since you're not declaring what your reference point is (sea level, ground level, etc.) it's ambiguous at best. Elevation tends not to have this problem, as while it's a relative measure, it's reference point is usually the Mean Sea Level.

Also from WP:

Although the term altitude is commonly used to mean the height above sea level of a location, in geography the term elevation is often preferred for this usage.

If you'd prefer to stick with altitude, you could add the modifier True to indicate that you're referring to the altitude above sea level.

See the Altitude page on WP for more info.