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

Mon Feb 8 21:20:05 GMT 2010

Hi Apollinaris,

> Hi Mark,
> 
> I think I have an example where this is useful. On a ~200km drive I got one
> keep left announcement where there was only a normal exit from the motorway.
> 
> Before blindly adding this relation  I want to understand how the current
> algorithm should work. maybe I should fix the osm data first
> you mention mkgmap looks at ways, ref, names. can you explain how it's done?

Sure. What it does is at each junction, it tries to determine what is
the "main road" and what is the "side road". If it can determine that,
it then adjusts the angle at which the side road leaves the main road
such that it reduces the likelyhood of getting keep left/right
instructions when you are staying on the main road and increases the
likelyhood of getting turn left/right instructions when you want to
take the side road. 

To determine which arcs make up the main road, it looks at some
attributes of the routing arcs that enter and leave that node. If two
arcs have the same ref, it's considered a strong indication that they
make up a pair, likewise, matching names are considered good.

The through_route relation is there to help the situation where it's
not possible to determine from the names/refs of the roads which is the
main road.

Before you change any data, could you please post the OSM URL of the
exit so that I can look at it and see if I can tell why you got a "keep
left". I am guessing, but I wouldn't be surprised if the exit ramp was
tagged as a road (motorway, trunk, etc.) and not as a ramp.

> 
> btw it's much better than my old tomtom which announces keep left on too
> many exits.

Good!

Cheers,

Mark