[Tagging] Feature proposal - RFC - ele with units

[Minh Nguyen]

Vào lúc 15:48 2021-12-06, Greg Troxel đã viết:
> 
> Minh Nguyen <minh at nguyen.cincinnati.oh.us> writes:
> 
>> And now for something unrelated to route relations. I've written up a
>> proposal that would:
>>
>> * Tweak the definition of the ele=* key to accept values in feet, as
>> long as the unit is specified, just like height=*, maxheight=*, etc. A
>> bare number without a unit symbol would continue to be in meters.
> 
> That seems ok.  You said "feet" so presuambly that is "international
> feet" rather than "US survey feet".  Although, the difference is so
> slight, and vastly smaller than the differences in datum and the likely
> errors that it does not matter.
> 
> For the record, I think ele in osm is "WGS84 Orthometric Height", which
> means more precisely:
> 
>    Start with ellipsoidal height in the currently-active realization of
>    WGS84 which is WGS84(G2139).
> 
>    Appply the geoid model associated with the currently-active WGS84
>    realization, which is EGM2008.
> 
> which leads to something that feels like "height above sea level", and
> is close but not exactly equal to height expressed in national datums
> like the US NAVD88 and the European EVRF, but is close enough for
> people that are not able to measure height to 25 cm.
> 
> (If you are someone who thinks you can measure height to 25cm or
> thereabouts, please reach out in private mail; I'd like to compare
> notes.)

These are valid considerations, but to the extent possible, I'd like to 
keep them out of scope for the proposal at hand. In fact, the proposal 
doesn't explicitly call for feet. Instead, it calls for consistency with 
OSM's general units standard. [1] If there's a need to distinguish 
between the international foot and the U.S. survey foot -- which will be 
essentially obsolete for elevations in a year's time -- then we should 
discuss that separately as part of the units documentation.

In my opinion, the ele=* key's documentation is overspecified. It says 
the key is based on EGM96 [2], but I would be shocked if 5% of the 
people who've manually added this key even know what that is. My most 
recent run-in with the key was when mapping a high water mark signposted 
in "feet above sea level" to the hundredths place from the comfort of my 
armless chair. (If I had been there in person, I would've added a 
source=survey tag for irony.)

The renderers that use ele=* have already made their peace with the fact 
that anything goes with respect to the reference or elevation model. 
I've linked the proposal to some other proposals for clarifying these 
details in separate keys.

[1] https://wiki.openstreetmap.org/wiki/Map_features/Units
[2] https://wiki.openstreetmap.org/wiki/Special:PermanentLink/2223781

-- 
minh at nguyen.cincinnati.oh.us