Colin Smale <colin.sm...@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and it seems that the European more modern height datum has already happened: https://epsg.io/5730 (superceded) https://epsg.io/5621 (European Vertical Reference Frame 2007) This looks useful: http://euref.eu/documentation/Tutorial2015/t-04-01-Liebsch.pdf And this all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. _______________________________________________ Tagging mailing list Tagging@openstreetmap.org https://lists.openstreetmap.org/listinfo/tagging
