When does ele become useless on areas?

[emersonveenstra (Emerson)]

Key:ele - OpenStreetMap Wiki is pretty vague when explaining what elements to add ele to, only saying to not tag objects where the elevation “is not an information of general interest”.

Looking around, i’m finding objects like Relation: ‪Michigan Islands National Wildlife Refuge‬ (‪8831276‬) | OpenStreetMap and Way: ‪Área Natural Municipal de Manejo Integrado Lagarpampa Molle Pampa‬ (‪230067527‬) | OpenStreetMap, which I would say the ele tag on those is definitely useless. But I want to make a clearer definition of where the line between useful and useless is for areas, so I want to get feedback on adding this to the wiki:

“Elevation should only be tagged on areas if the elevation variance across the entire area is less than the margin of error of the elevation source.”

Would this be a good thing to add? Is it too rigid of a rule, or is the wording too confusing?

[ZeLonewolf (Brian M Sperlongano)]

For tags which have been used very widely, it is not possible to add rules ex post facto. You can only describe how mappers have used it.

[dieterdreist (Martin Koppenhöfer)]

ok, but a single elevation value on a way or area only makes sense if the area or way is flat (roughly on the same elevation), this is implicit, and is a requirement from verifiability.

[Minh_Nguyen (Minh Nguyễn)]

In parts of the U.S., it’s customary to associate a single elevation with a whole city regardless of topological variation within it. These elevations are posted at the city limits and at train stations.

@stevea

@jumbanho

Offtopic: I love when hilly cities have a single elevation precise to the one foot level. (edit: excuse me, city counties)

Offtopic: As a child who loved maps, my father explained this to me (I was five or so?) by saying “that figure is usually the elevation of City Hall, or sometimes the train station.” And I discovered both that he was (mostly) correct, and what is known as a “mapping convention.” (The method by which something cartographic is customarily done, not an annual SOTM-US, for example).

@Minh_Nguyen

For roadside jurisdictional signs, like the San Francisco sign on the Golden Gate Bridge, the California MUTCD requires the elevation to be the spot elevation at “the courthouse, post office, railroad station, or benchmark in the central district of the city”. Which one it is for San Francisco is left as an exercise to the reader. (The standard for city elevations on railroad station signs may be different.)

Ideally, this elevation would be tagged on wherever the elevation was measured, but this location isn’t always obvious to us. Tagging this elevation on the place=city point may be misleading, as it implies a spot elevation at that point. Tagging it on the boundary relation that embraces the spot elevation would be less misleading, if a bit weird.

Anyways, the ele=* tag on the National Wildlife Refuge didn’t come from a city limit sign, or any sign for that matter. It’s a spot elevation from GNIS, based on an elevation model at a specific location where old USGS topographic maps labeled the refuge. We imported the refuge from GNIS as a point at that location, but later the point was conflated with the boundary relation. Since the spot elevation comes from an arbitrary spot on an elevation model, and the refuge has a diffuse geometry, the ele=* tag should’ve been deleted at that time instead of being copied over.