Alan Mintz <[email protected]> writes: > This is the standard for FCC (communications) and FAA (airspace) in > the US. Well, close at least - elevations are generally "above mean > sea level" - I don't know how that relates to the WGS84/GPS and/or > survey elevation but I'd expect them to be close.
"above mean sea level" (not claiming the FCC doesn't use it; I've seen it too) is basically sloppy from a surveying/geodesy viewpoint; the notion of mean sea level only applies at a specific tide gauge. The modern concept is orthometric height relative to a given vertical datum (in the US, NAVD88). This is more or less the same conceptually as MSL except that it doesn't presume that mean sea level at all tide gauges is the same (as NGVD29 did). Orthometric height is based on gravity, rather than geometry, and is more or less distance normal to the geoid, a surface of constant potential that sort of matches sea level. WGS84 proper measures locations relative to the ellipsoid. One would refer to the measured height (transformed to lat/lon/height from XYZ in earth-centered earth-fixed) as an ellipsoidal height. But because what everyone wants is orthometric height (partly because of tradition and existing data, and partly because water flows downhill relative to orthometric height), one uses a geoid model that estimates the distance From the ellipsoid to the geoid. From that one gets an estimate of orthometric height. On every GPS receiver I've seen, the "altitude" is intended to match orthometric height, and is ellipsoidal height adjusted by the geoid model. So it's ok to talk about WGS84 elevations, but we should be clear that we mean elevations intended to be usable as orthometric heights using the geoid model. ellipsoid/geoid separates are large; around me it's ~30m. But errors in geoid models and differences in semi-modern (20th century and newer) vertical datums are a meter or so, at least in North America. So despite my ranting, this is mostly ignorable for OSM.
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