[mkgmap-dev] What is the idea behind --adjust-turn-headings?

Fri Aug 7 10:22:25 BST 2015

On Thu, Aug 06, 2015 at 03:11:23PM +0200, Gerd Petermann wrote:
>I understand that this option should help to produce good driving 
>instructions. Anyhow, I wonder if the result is really better.
>
>The two screenshots show the difference at this node:
>http://www.openstreetmap.org/node/21725099

This node is a 3-way T-shaped crossing, with roughly 90° angles between 
the 3 roads.

If my memory serves me correctly, the motivation behind 
--adjust-turn-headings was to help in cases where you are entering a 
Y-shaped crossing from the bottom, and there is a very sharp angle 
between the branches of the Y. At the extreme, one of the branches is 
going straight, and the other is at some very small angle. In this case, 
you might want a direction such as "turn left" or "turn right", if 
Garmin cannot produce a "keep left" or "keep right" direction.

IIRC, without the option, when entering this cycleway ramp from the 
north:

http://www.openstreetmap.org/node/33986539

the Garmin Edge 705 would not give any announcement that I have to turn.  
With --adjust-turn-headings, it would tell me to turn left. I do not 
have the device any more, so I cannot test this.

>Before I can fix the additinal problems introduced by the naming of 
>service roads I'd like to understand if this is the intended behaviour.

I would say that it is not. If you have a T-crossing with the nodes AC 
at the end of the horizontal section of the T and the node B at the 
bottom, and you are going left to right from A to C, the road names or 
the way how the roads are split to way segments should not matter. It 
also should not matter if the route A-C is bending slightly.

IMO, what should matter is the difference of the angles of the roads.  
In this case, the angle difference between A-C and A-B is about 90°. If 
that angle difference were smaller than some threshold (say, 45° or 
maybe even 30°), then it would make sense to adjust the turn headings.  
Otherwise the turn directions would be simply noise.

	Marko