Basically I think the whole elevation as height above ellipsoid is mostly a huge misunderstanding, and I wonder how much support there is for it.
My memory matches what Martin pointed to: ele= is "height above sea level". And, given that layman's terms description, and that OSM is using WGS84, then it seems really obvious that the particular flavor of "height above sea level" should be "WGS84 altitude" meaning "height above the WGS84 geoid". In the US, ele= tags typically have the NAVD88 height, from GNIS, or elevations from GPS receivers that do geoid corrections (e.g. Garmin). (Yes, there's slight fuzz but it's tiny compared to vertical GPS accuracy.) The talk page you pointed to says "elevation should be WGS84 because we use GPS" and then reasons that this must be HAE, even though almost no GPS receiver used by a mapper (pre android) would have been showing HAE. In fact, to this day I have not seen a GPS receiver (other than geodetic survey type ones) display height in HAE (without labeling it as such), other than buggy android apps. So it's likely that people were using what their receivers displayed (height above geoid) and perhaps just thought that was HAE, rather than really intending to use height values with huge differences from conventional height datums. Stepping back from untangling the history, I have three questions about ele=: 1) Does anybody think it makes sense to use HAE in OSM, vs "WGS84 elevation" meaning heigth above geoid? 2) Are there lots of points in OSM that actually have ele in HAE? (I am unaware of any such points in the US.) 3) Are there any national height datums that are ellipsoidal height? (I think not, because height is very much about water and therefore about gravity.) If not, why is it sensible for OSM to start doing this? IMHO, OSM should follow existing standards and practices of the relevant fields in terms of definitions, to the extent that this is sensible. So I would propose: adjust ele page to be very clear that this is WGS84 altitude (height above geoid) and caution about bad data on some android apps. mark Altitude page as an old proposal that has no current support and if that's off base it would be good to hear from people how it's off, and why. _______________________________________________ Tagging mailing list Tagging@openstreetmap.org https://lists.openstreetmap.org/listinfo/tagging
