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@iki.fi To: mkgmap-dev@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.
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