Sea Level

Sea level refers to the surface. Since the level of the water surface is uniform throughout the planet, we measure land features in relationship to this uniform level. Thus all mountains are measured as being so many feet or meters above sea level, rather than being measured from base to summit. Some valleys on earth are so sunken that they are actually below sea level. They don't get flooded because they are nowhere near the sea.

The ocean surface has highs and lows, similar to the hills and valleys of Earth's land surface depicted on a topographic map. These variations, called "ocean surface topography" or "dynamic sea surface topography" are mapped using measurements of sea surface height relative to Earth's geoid. Earth's geoid is a calculated surface of equal gravitational potential energy and represents the shape the sea surface would be if the ocean were not in motion.

The height variations of ocean surface topography can be as much as two meters and are influenced by ocean circulation, ocean temperature, and salinity.

http://en.wikipedia.org/wiki/Ocean_surface_topography

Kewl.

Never thought of the oceans as having topography before.

Still, I doubt anyone has splatted out against the runway because they were using the altimeter to decide when to flare out. Variations in barametric pressure would make the things more unreliable than any variation is sea level.

"Sea level" should be correctly "mean sea level" ... as far as I remember classes in the naval college, at least

Here, in (Western) Europe, we use the NAP (Normaal Amsterdams Peil or Amsterdam Ordnance Datum). The wikipedia article gives some ideas how that level was calculated ...

The ocean surface topography is something (slightly) different.

Sheesh . . . anyone who's ever been to sea knows it ain't level . . . in fact, it can't sit still . . .

Yeah, if the sea was level, I would never have gotten seasick. One day I stood watch, looking out for sea traffic. There was a fairly rare trough that opened between the ship and another one out there. It happened quickly enough that I could easily have not noticed. The officer I reported to warned me to not be taking sightings from radar.