Although I’m working over 15 years with digital map data I’m pretty new to OSM.
What I consider as real good and useful idea are the information in Open Fire Maps. Unfortunately the hydrants in our village are not in. If I get the data from the municipal administration, is there a way to create a XML or CSV (maybe also Json, but there I’m not really familiar with yet)-file to import all hydrants at once?
Where do I find a description how to indicate (in this case) that it’s an upper or lower version and which diameter it has?
I don’t know what “lower and upper versions” mean, but generally in OSM, you should search the Wiki and TagInfo to see if / how others already tag this. If there are no existing tags for it, you can create your own tag and you should document it in the Wiki, See Any_tags_you_like. Prior discussion (typically at the tagging mailing list) is recommended.