[Tagging] Elevation and height on vertical features

Colin Smale colin.smale at xs4all.nl
Fri Jan 8 11:11:07 UTC 2016


So if all the elevations in OSM should be interpreted as WGS84, but many
(most?) of them are not, we have no way of knowing which are "right" and
which are "wrong". Even if the numerical value of ele=* is correct, we
have unreliable data. 

Where do we go from here? Maybe we should encourage an explicit
reference to the datum used, such as ele:wgs84=* or ele:osgb=*? Or
should we heuristically assume that no mappers will have translated
elevations to wgs84 anyway, and change the wiki to make the definition
into "national standards"? 

According to wikipedia [1] the differences between the datums can be up
to 100m.

--colin 

[1] https://en.wikipedia.org/wiki/Sea_level#Height_above_mean_sea_level 

On 2016-01-08 12:00, Martin Koppenhoefer wrote:

> 2016-01-08 2:30 GMT+01:00 Warin <61sundowner at gmail.com>:
> 
>> Grasping at straws .. the elevation of a mountain is given as its peak. If there is consistency within the map then the elevation of all objects should be their maximum height.
> 
> don't confuse elevation and height. Elevation is typically referring to the ground. For a mountain, the situation is clearly different, it is the peak we are mapping, not the mountain, because it is not even clear what a mountain is (where it starts).
> 
> We will also need to standardize on a datum for elevations. The wiki refers to both mean sea level (which varies by country) and wgs84. The differences might be enough to take the wheels off your plane.. Good point there! :-)     
> For most it won't matter. What do international planes use as there reference for height? Use that - again consistency.

why planes? If you fly a plane you have to have specific airspace maps,
nothing where OSM is particularily useful. Normally, maps are using
elevation indications in the national system. AFAIK, we have already
standardized elevation indications: they're * in WGS84 like all our
data. 

Cheers, 
Martin

* well, they should be, typically the mappers read the elevation off
some sign and don't bother to convert. 

PPS: Sorry, it isn't actually clear that WGS84 should be used, I have
just discovered that the "meters above sea level" indication has already
been introduced in 2008 in the post-vote-cleanup phase ;-)
http://wiki.openstreetmap.org/w/index.php?title=Key%3Aele&type=revision&diff=151029&oldid=125595

  
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