Me: Oh cool, there's some elevation data available on this creek. I bet OSM has a simple way to represent that!
OSM elevation docs: If you use the WGS84 ellipsoid instead of the EGM96 geoid then you are worthless to me.
ME: What? Oh okay, there's a "basics" section...
OSM elevation docs: Vertical datums other than the EGM96 geoid can use ele:xyz=* key value pairs.
Oh it didn't actually matter much, the points were just at two ends of a long straight tunnel (which I thought I'd add in case someone wanted to know what angle it flows at, which doesn't even need absolute reference). My point was more that a priori, particularly for new users, I could imagine it being hard to understand why some tags are simple to add (e.g. address, website) while others like elevation or hours of operation require real documentation to use correctly
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