[Tagging] RFC ele:regional

[Greg Troxel]

Martin Koppenhoefer <dieterdreist at gmail.com> writes:

> I’m asking for comments on https://wiki.openstreetmap.org/wiki/Proposed_features/ele:regional

Two big comments:

  First, the current wiki documentation about ele and Altitude should be
  really straigthened out, so that we have a basis for what we are
  comparing to.

  Second, the notion of a single regional vertical datum doesn't really
  work.  In the US, that could be the old NGVD29, or the current NAVD88.
  Plus, we are about to get NATRF2022.  However, all of these are within
  a meter or so, and in terms of vertical data in OSM, that's not really
  a big problem.  So if there is going to be precision, then we should
  follow GIS and have an explicit datum.  I would say an EPSG code is
  sensible -- see the proj package for canonical values.



As for ele/Altitude, there is great confusion out there about "WGS84"
and two separate concepts:

  height above the ellipsoid.  Often written HAE. The ellipsoid is a
  mathematical surface that is NOT a surface of equal gravity.  While
  geodesists and geodetic surveyors use it, and while it's part of the
  calculations within GPS, I am not aware of a single vertical datum in
  use in any country that is relative to the ellipisoid.  Note that
  water does not flow "downhill" when "down" means to a lower value of
  HAE.  Water is hugely important in elevation and mapping.

  height above geoid, or height above a reference equal-gravity surface,
  or height above sea level.  (Yes, I realize that "sea level" is a huge
  can of worms.)   This is more or less what every height system uses or
  intends to use.


In WGS84, one gets from the base computation lat/lon and a height above
the ellipsoid.  This is purely a geometric answer and is totally
disconnected from grravity.  Then, GPS receivers use a gravity model to
compute the offset from the ellipsoid and the reference gravity surface
(which is more or less the "sea level surface"), and they them use that
to get a "height above sea level".  Receivers that display to humans
display this sea level height.  NMEA has that same sea level height.

(Android stands alone in that the API returns height above ellipsoid.
That's not wrong, but it is unusual.  IMHO how Android defines an
interface is irrelevant to OSM's definitions.)


When people say "WGS84 altitude", they mean the height above the WGS84
equal-gravity surface as computed from the ellipsoidal height and the
gravity model.  This is sort of 0m at sea level.  Note that the
ellipsoid can be 100m different from this equal-gravity surface, and is
often significantly different.  It's ~30m in Boston and I hear it's 50m
in Switzerland.  Nobody who says "WGS84 altitude" really means "WGS84
ellipsoidal height".  If they did, they would say "WGS84 ellipsoidal
height".