[Tagging] RFC ele:regional

[Greg Troxel]

Colin Smale <colin.smale at xs4all.nl> writes:

> As I mentioned before, the national datums of the Netherlands and
> Belgium differ by over 2m, which for everything connected to water is
> very significant. Waterways often form the border, with bridges that
> cross the border. You cannot use a map/chart (at last for tidal waters)
> if you don't know what datum it uses. 

Thanks.  2m is perhaps significant, and I'm surprised it's that much.  I
would suggest that if people care about that 2m, then they need to
transform to a common height reference.

I would expect that Europe is working on a new satellite-native regional
height system, similar to the new 2020 in Australia and the 2022 one in
North  America, that will basically align heights.

> In OSM we often leave out "obvious" annotations, giving rise to a kind
> of "default" (such as maxspeed in km/h). But there is always a way of
> making it explicit, for those who feel the need. In this case we may
> agree to define "ele" as relative to the "local datum" or WGS84 or
> whatever, but we must always provide a system for making that explicit,
> and (preferably) a means to derive the intended basis for values that
> are not explicitly qualified.

So what do you think about what I've been saying:

  ele is assumed to be in, and should be represented in, WGS84 height
  above geoid (as the international norm, aligned with OSM's horizontal
  datum)

  ele:datum=unknown represents that the mapper doesn't know what datum
  the number they put in ele is expressed in

  ele:datum=EPSG:5703 (as an example for NAVD88), when the mapper really
  does know the datum, and doesn't want to transform to WGS84


I assume you are referring to:

  https://epsg.io/5709

  https://epsg.io/5710

and it seems that the European more modern height datum has already
happened:

  https://epsg.io/5730 (superceded)
  https://epsg.io/5621 (European Vertical Reference Frame 2007)

This looks useful:

  http://euref.eu/documentation/Tutorial2015/t-04-01-Liebsch.pdf

And this all makes me think that an elevation without a datum cannot be
reliably used at the better than 2m level.