[Proj] Geodesic distances away from the ellipsoid

support.mn at elisanet.fi support.mn at elisanet.fi
Wed Jul 15 03:30:45 EST 2009

strebe [strebe at aol.com] kirjoitti: 
> Presumably constant height is of interest for... flying? Or...? (I cannot imagine any practical use for a precise solution, but there's no reason that should shut off consideration.)

Not exactly,
they are talking about flight levels and that is defined as:

"A Flight Level (FL) is a standard nominal altitude of an aircraft, in hundreds of feet. This altitude is calculated from a world-wide fixed pressure datum of 1013.25 hPa (29.921 inHg), the average sea-level pressure, and therefore is not necessarily the same as the aircraft's true altitude either above mean sea level or above ground level."


The WGS84 ellipsoid and the average sea level can differ several hundreds meters. So to be exact one should first calculate the geoid height and then transform that to an ellipsoid height. And even adjust for the fact that the actual sea level pressure at that point is far from the average.

Janne. / MNS Support

More information about the Proj mailing list