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

Fri Aug 7 11:00:25 BST 2015

Hi Marko,

thanks, I think the 2nd block in the comments for the code suggest something else:
            // detect the "shallow turn" scenario where at a junction
            // on some "main" road, the side road leaves the main
            // road at a very shallow angle and the GPS says "keep
            // right/left" when it would be better if it said "turn
            // right/left"

            // also helps to produce a turn instruction when the main
            // road bends sharply but the side road keeps close to the
            // original heading

Your example matches the "shallow turn" scenario, I think my example 
matches the 2nd one. 

I do not yet understand the effect of the parameter (1/2/3) for the option, I guess
I have to trace some more examples...

Gerd

> Date: Fri, 7 Aug 2015 12:22:25 +0300
> From: marko.makela at iki.fi
> To: mkgmap-dev at lists.mkgmap.org.uk
> Subject: Re: [mkgmap-dev] What is the idea behind --adjust-turn-headings?
> 
> 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
> _______________________________________________
> mkgmap-dev mailing list
> mkgmap-dev at lists.mkgmap.org.uk
> http://www.mkgmap.org.uk/mailman/listinfo/mkgmap-dev

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.mkgmap.org.uk/pipermail/mkgmap-dev/attachments/20150807/3fd1b88f/attachment.html>