Am 27.09.2016 um 22:21 schrieb Markus Bärlocher:
The "elevation" property we have (P2044) is defined to refer to NN
It is not a good idea, to define 'elevation' like it is "defined" in P2044: there are hundreds of reference-levels (not only NN)...
Yes, I agree. But that's how it currently is. You can start a discussion about it on the property's talk page, or on the project chat page, or some other appropriate place.
Then you could express something like "elevation: 28.3m;
In WD there is a confusion between altitude and elevation? (may be in American and British English? or geographic and aviation and astronomy?)
As far as I know, WD uses "altitude" only as an alias of "elevation". I'm not a native speaker of English, but as far as I know, you can use "altitude" as well as "elevation" when describing a geographical point. The definitions of the corresponding items (Q190200 and Q2633778) reflect that, and so do the definitions in Merriam Webster. However, "elevation" seems to be used only for fixed places - a plane has altitude, not elevation - so that's a reason not to merge the two items.
However, if I understood correctly, what you are looking for is actually not elevation, but "clearance" ("Lichte Höhe"): https://www.wikidata.org/wiki/Q1823312. Interestingly, there is also Q2446632...
Oh, we actually do have a property for that! P2793 is the "distance between surface and bottom of a bridge deck". That's exactly what you need, no?
But this is a combination of unit and reference-level: 'm ü.M.'
We should not mix or confound this modellings...
What will be the WD-way? (you should discuss this with a geodetic specialist...!)
Indeed :) And a civil engineer. But for starters, maybe Aude has some thoughts on this.
Additionally we need an expression for 'accuracy' and 'source': If the hight unit is 'meter' and the source value is in 'feet', the new value could have a lot more/less digits than the source, but no better/worse accuracy...
Sources can be given for any statement. Accuracy can be qiven for any quantity value, just enter 32+-2m. If the source gives the number in feet, please enter it in feet in Wikidata, and leave the conversion to the software (we are just in the process of adding support for unit conversion)
HTH, Daniel
PS: I'm a software guy. I know how Wikibase and MediaWiki work, and I know the underlying data model of Wikidata quite well. But I do not know all the properties and conventions, and I may not be aware of the best place to discuss these things. So please don't rely on my opinion about modeling on Wikidata too much.