[Tagging] RFC ele:regional

[Colin Smale]

On 2020-05-08 14:09, Greg Troxel wrote:

> Martin Koppenhoefer <dieterdreist at gmail.com> writes:
> 
> Am Fr., 8. Mai 2020 um 03:22 Uhr schrieb Greg Troxel <gdt at lexort.com>:
> 
> 3) Look up the data sheet and mark it as ele:datum=NGVD29 or
> ele:datum=NAVD88 as it turns out. 
> IIRR, in another mail, you wrote that the difference between these 2 is
> less than a meter, can you confirm this, or did I understand you or
> remember wrong?

Yes,it typically is.

So let me ask you again, since you keep declining to answer this:

  Please give an example of an elevation of a real thing that is
  meaningfully different in one of these "regional datums" (established
  by a country's survey agency) compared to WGS84 height above geoid.
  Identify the regional datum, and identify two values linked with a
  rigorous transformation (such as national survey agencies publish). 

As I mentioned before, the national datums of the Netherlands and
Belgium differ by over 2m, which for everything connected to water is
very significant. Waterways often form the border, with bridges that
cross the border. You cannot use a map/chart (at last for tidal waters)
if you don't know what datum it uses. 

In OSM we often leave out "obvious" annotations, giving rise to a kind
of "default" (such as maxspeed in km/h). But there is always a way of
making it explicit, for those who feel the need. In this case we may
agree to define "ele" as relative to the "local datum" or WGS84 or
whatever, but we must always provide a system for making that explicit,
and (preferably) a means to derive the intended basis for values that
are not explicitly qualified.
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