Originally Posted by
DaveReidUK
Doesn't GPS actually measure relative, rather than absolute, altitude - that's to say relative to a reference ellipsoid that approximates the earth's surface ?
I think there are several datums that are available for reporting, including the GPS default ellipsoid WGS-84 (apparently), but a GPS nav unit is initially calculating a 3D position based on the orbits of the GPS satellite constellation. The report should be consistent for a consistent location, such as a if one is on a radio tower or mountain peak. The pressure altitude will vary and the difference with some other fixed location will vary for those fixed locations.
The more I look at it the more I don't want to look at it.
https://geodesy.noaa.gov/GEOID/index.shtml It starts to glaze over the brain when considering that we live on a lumpy blob that is changing shape in ways that are measurable, but not reliably so from any location on the planet. Worse, where the satellites are is affected by the lumpy gravity of that lumpy blob. Gah!
There appears to be a project to convert to using the constellation directly as the reference; there is also a project to generate and equi-potential (they call it orthometric) model that includes differences in local gravity that is very useful in predicting water flow and flood plains.
https://geodesy.noaa.gov/GRAV-D/pubs...2007_12_19.pdf