On 19.09.2016 19:02, Denis Stein wrote:
> P1. Proposals for the position of tags for single turnouts
>     (abbreviation: ST; including both straight as well as curved ones):
> P1(a) Tip of the switch blade (i.e., the position where the route
>       changes; might be also recognized by the point machine in aerial
>       images)
> P1(b) Tip of the frog ("massif" part; can be approximated from the
>       intersection of both inner rails)
> P1(c) another proposal?
> P2. Proposals for the position of tags for diamond crossings
>     (abbreviation: DC):
> P2(a) Center of DC, i.e. intersection of both track centerlines
> P2(b) another proposal?
> P3. Proposals for tagging of complex switches:
> P3.1 Decompose them whenever applicable:
>      - diamond crossing with ... slips:
>        * single = 2x ST + 1x DC
>        * double = 4x ST + 1x DC
>      - three-way switch = 2x ST
>      (solves also the route ambiguity problem in single slip case)
> P3.2 Tag those parts according to P1 and P2.
> _My favorite_ is: P1(a) + P2(a) + P3.1 + P3.2, which considers the
> position of turnouts and diamond crossing and clearly defines the
> drivable routes on them clearly.

Yes, that is what I tagged in the past and it's also my favourite
because it's suited best for routing purposes as well as collision
detection (in conjunction with track gauge). Especially because for
roads we also tag the centerline, so there is the principle of least
surprise, and algorithms can be reused for railways.

I currently cannot think of any opposing arguments, maybe because I'm
used to thinking of infrastructure in a topological way.

 - Roland
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