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I'm using OSM maps on my Garmin Nuvi and have noticed some strange routing issues that seem to be related to feeder roads on roundabouts.

Assuming exit to the North is the assumed best route...

If the on/off feeder is created as a single way, highlighted as below, I think the routing algorithm is seeing this as at least inefficient, doubling back, and finding a different route.


I can't find any info on this style so would appreciate any opinions.

This may be specific to Garmin routing or even routing algorithm used to generate the maps so of little relevance to OSM.

asked 01 Sep '12, 08:19

fruit-uk's gravatar image

accept rate: 25%

edited 01 Sep '12, 08:21

There shouldn't be any difference for any routing algorithm and any end device as long as the data pre-processing is done correctly, and I doubt that mkgmap (used for creating maps for Garmin devices) cannot handle this situation correctly.

Could you tell us where this happened, which of the various available Garmin maps you used and which start and end point you choosed for the route? You should also try different maps to check if all of them have this problem or if it is just a bug in a single map. In the latter case it is probably the best to contact the author of this map directly.

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answered 01 Sep '12, 10:46

scai's gravatar image

scai ♦
accept rate: 23%


If it's a map that you've created yourself (and therefore you know the parameters that you passed to mkgmap to create the map) then it might be worth asking on the mkgmap development mailing list. Mkgmap does have a large number of parameters that control e.g. routing (including the delightfully named "frig-roundabouts"), so without more info on how the map was created it would be difficult to comment.

If we could find out from and to where that the problem occurs, then people may be able test with their own generated maps.

(01 Sep '12, 11:31) SomeoneElse ♦

I've found time to look at this more closely today.

There doesn't seem to be an issue with connected flares and routing after all, the problem was a short section of road wrongly tagged oneway.

Guess this thread can be closed now.

Incidentally, I did come across a reference that appears to suggest flares should be separate ways in documentation of a warning generated by mkgmap

Incoming/Outgoing roundabout flare road (...) does not start/finish at flare?

but I've no idea how relevant that is to OSM in general.

(04 Sep '12, 09:40) fruit-uk

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question asked: 01 Sep '12, 08:19

question was seen: 3,884 times

last updated: 05 Sep '12, 17:00

NOTICE: is no longer in use from 1st March 2024. Please use the OpenStreetMap Community Forum