[mkgmap-dev] Problem with --adjust-turn-headings

Fri Feb 5 14:43:38 GMT 2010

Mark Burton wrote:
> Hi Steve,
>
>   
>> Please look at 
>> http://www.openstreetmap.org/?mlat=51.44827&mlon=-3.49581&zoom=16&layers=B000FTF
>>
>> If you approach the marked junction on the B4270 from the south, 
>> intending to continue northwards on the B4268 then you get told "turn 
>> left" as you approach the junction with 300m or so to go. Trouble is, 
>> there is a minor road junctioning with the B4270 at about the same point 
>> on the road, and the geometry of the combined junction most certainly 
>> does not agree with a "turn left" for the B4268!
>>
>> The portion of the B4270 that continues to the north-east makes a 
>> junction with the B4268/B4270 as a "give way" on the road. The 
>> transition from the B4270 to the B4268 is seamless on the road. The 
>> minor road going west does of course join the B4270 as a "give way" on 
>> the road. You can see the physical details on Google Earth if you need 
>> more detail.
>>
>> But to hear "turn left in 300m" is most certainly a surprise! I'd expect 
>> no verbal instruction in this case.
>>     
>
> Yes, that's not ideal is it?
>   
:-)
> Tricky case this one. The proximity of the minor road on the left to
> the bigger junction does confuse the issue. However, [...] the GPS doesn't know that the road continues in a straight line
> and that the turn to the NE is in fact the side road.
>
> Now, if the OSM data could actually express the fact that the straight
> on direction was the "main road" (even though the road to the NE has
> the same ref as the incoming road), then we could avoid this problem.
>
> Perhaps we should introduce a relation type (prefixed with mkgmap:,
> perhaps) that could be used in cases like this to express that two ways
> are "the same road" even though their refs are different.
>
>   

I was going to suggest that myself, so that's two of us at least!

However - I'd not suggest prefixing with "mkgmap:" because probably 
every other user of OSM data to create routing instructions would also 
be glad of such a mark-up. I guess that the mkgmap community could just 
agree amongst ourselves to try out such an idea, and once it proves 
itself to be useful, get it ratified by the main OSM people?

Always easier to ratify something that is know to work and be useful 
than to try and ratify a wild idea that's not been tried!

You're the guru, Mark, what do you suggest as a relation, and do you 
reckon you could implement such a thing "just for a try-out" anytime soon?

I'd think that maybe a relation called "throughway" or "thoroughfare" 
would be the thing to use, and get it just to list the ways that make up 
the major-route from the P.O.V of a road-user. "Thoroughfare" is a bit 
hard to type though, though at least in English it should be fairly 
obvious what it is supposed to mean (Anglo Saxon roots there?). I'd also 
allow such a relation to plough straight through several junctions if 
necessary, indicating the primary way for all of them, to save having to 
split ways up and use different relations to disambiguate a multitude of 
physically-close junctions.

99% of junctions won't need them, but pathological cases will always 
exist - as that one did.

Steve

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