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[mkgmap-dev] Strange routing issue when joining a "through_route"

From Steve Hosgood steve at stoneship.org.uk on Wed Oct 19 10:10:01 BST 2011

The following oddity happens around node 86872929 (lat 51.448272, lon 

I've got yesterday's maps running on a Streetpilot i3. I approach the 
problem area from the west on unclassified road (way 22883317) expecting 
to go south on the secondary route B4270 (way 4597013). Normally I'd 
expect to be routed onto the short link of unclassified road (way 
22883320), so I'd expect the driving instructions "turn right, then turn 
right" as I approach node 246057790.

What I actually got on approach was "in 100m, turn left". Looking at the 
Garmin's screen (which was already at full-zoom) it seemed that the 
machine wanted to turn me north on the B4270 and do a u-turn at node 
86872929. Normally a Streetpilot will try and do everything it can to 
avoid u-turns, but not in this case.

It seems as if way 4597013 (which happens to be part of a 
"through_route" relation) is being treated as a one-way street going 
north for the purposes of junctions that meet with it. Node 86872929 
(where the u-turn is flagged) is also in that relation with the role of 

The "direction" of way 4597013 is however being shown as north to south, 
so treating it as a one-way in the opposite direction would be a bit weird.

This "through_route" info may be a red herring, but I've never seen such 
behaviour elsewhere, and it's the only unusual feature of this junction 
that I can see.


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