[OSM-talk] Removing redundant routing instructions

Colin Smale colin.smale at xs4all.nl
Sun Apr 26 13:52:37 UTC 2015


The difference between routing and navigation is that the routing
algorithm will work out which road you need to be on, but it is the
"navigation" aspect which makes translates the routing graph to useful
instructions for a human. If the main road does a 90 degree left at a
T-junction, something has to work out whether to say "follow the road
through the bend" or "turn left" or something else or nothing at all.
Anyone who implies that geometry alone is enough to make that decision,
is confusing the two concepts IMHO. 

Arguments that it is not needed for routing are possibly correct, but
short-sighted. The "through_route" idea doesn't change anything for the
routing (although routing algorithms may add a time penalty for
give-ways or sharp bends if they wish) but it adds a hint as to how best
to describe the next "move" for the driver. 

A motorway exit is also an "aid to navigation" in that it affects only
the (spoken) instructions, and not the route chosen by the routing

I would be in favour of reviving the old proposal. 


On 2015-04-26 15:26, Rob Nickerson wrote: 

>> There already is a "through_route" relation, to show the path of the
>> through route. It might not be well documented, but it is used (I
>> believe)by mkgmap. 
>> There was a proposal, which was eventually rejected:
>> http://wiki.openstreetmap.org/wiki/Proposed_features/through_route [2] 
>> IMHO it was rejected as it was seen as a hint for the router, not as an
>> aid to navigation, and people don't understand the difference. 
> Yeah I'm aware of that. In fact my example has been sat on my computer since that proposal and I've only just got back to looking at it!!
> This is in effect a revival of that proposal with a quite different example. I picked a different name as Through Route has a meaning in the UK - it means a route that takes you past a town whilst avoiding the congested city centre. If you think we should revive the through_route proposal then I'm happy with that instead.
> I'm not sure I get your point about "hint for router" versus "aid for navigation". I suspect this may stem from the don't tag for the renderer rule. If we look at the end use case the aim is to get a routing engine that provides an optimal route with user friendly route instructions. I can't believe this is an easy tag and as such I would expect the routing developers to be raising issues they cannot solve via code alone. This is one area where I would like my SatNav not to spew redundant instructions.
> Best, Rob
> p.s. Is highway=motorway_junction a "hint for router" or an "aid for navigation"? 
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[1] https://lists.openstreetmap.org/listinfo/talk
[2] http://wiki.openstreetmap.org/wiki/Proposed_features/through_route
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