Am 13.08.2020 um 19:06 schrieb Colin Smale:
On 2020-08-13 18:35, [email protected] wrote:
Hi,
in my opinion (i think dktue is right there) it should be easy for a
user to distinguish or extract (overpass query) the upper, mid and
lower stations.
That´s not possible at the moment in OSM. Elevation (ele tag) may be
useful, but does not indicate whether a station is the lower or upper
one.
On the other hand, a tagging with upper_station or lower_station is
clear and self-explanatory.
It would also be a denormalisation of the data. In SQL terms you would
use a subquery like "WHERE ele = MIN(SELECT ele FROM stations
WHERE...)" although I suspect the usual use case will be looking for
the whole line, so a simple "ORDER BY ele" would give you all you need
to know - bottom station is first row, top station is last row, rows
2..n-1 are mid stations. This would avoid any possibility of
conflicting information, e.g. multiple stations tagged as top, or the
station with the lowest elevation being tagged as top station.
All this is based on the assumption that the definition of the top
station is the one with the highest altitude.... If any other factors
are in play that could mean that this definition does not always hold
true, then I am keen to hear.... It's best to check assumptions, even
(especially?) if they do appear obvious.
I agree that this is denormalization, but: We still do this in OSM with
addresses and zipcodes for example and this is a good thing to do:
Because it allows to find potential errors in the data in an automated
way. I think it's very hard to find a wrong ele-Tag but it would be easy
to hint in a quality assurance tool that ele-Tag and station-Tag do
conflict.
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