[Tagging] Elevation and height on vertical features

Warin 61sundowner at gmail.com
Fri Jan 8 01:30:50 UTC 2016

On 8/01/2016 9:56 AM, Colin Smale wrote:
> Nobody will be using the raw data to fly a plane. It doesn't matter if 
> we use the ele tag for the top or the bottom - as long as the height 
> is given, the other value can easily be derived. What is important is 
> consistency, both in its definition and it's usage. Defining it as 
> sometimes the top and sometimes the bottom of a feature doesn't help.

And consistent with other mapping products/practices?

By convention maps uses blue to indicate sea, rivers and streams.
The idea is to be readily usable by using the same practice as others 
have used in the past.
  Not to set some new standard where there is no reason for it.

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.

> 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.

By maintaining some consistency with what has been done elsewhere it 
will make it easier to compare, check and confirm OSM data.
> On 7 January 2016 22:50:02 CET, Warin <61sundowner at gmail.com> wrote:
>     On 8/01/2016 3:32 AM, Christoph Hormann wrote:
>         On Thursday 07 January 2016, Aaron Spaulding wrote:
>             Hi all, I’ve been working on generating 3D meshes based on
>             OSM data and I ran into a problem. Vertical features like
>             'natural=cliff', 'barrier=retaining_wall’ and
>             'waterway=waterfall' occupy two points in physical space,
>             but because of the 2D nature of OSM its ambiguous which
>             side of the feature that the ‘ele’ tag applies. 
>         For cliffs mapping conventions say that you should put the
>         line on top of the cliff in case it is not exactly vertical -
>         accordingly the ele tag would also refer to the top - but keep
>         in mind that the elevation does not have to be constant.
>     Consider who is going to use the map, and for what purpose.
>     The most critical use is for aeroplanes .. where the maximum height is
>     critical information!
>     I think for that reason alone most maps should indicate the maximum
>     elevation of an object.
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