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Geometry mapping antipattern: Keep left

Posted Wed Dec 21 11:46:33 2022 Florian Lohoff

For some time i was thinking about making a blog post series about geometry mapping antipattern which i find often in our dataset. So here we come with No. 1. "Keep Left"

Have you ever wondered why you get stray "Keep left", "Keep right" announcements from your navigational system although you are on a straight road cruising ahead?

It because of geometry. All announcements concerning the driving direction are direct causes of the street geometry. So when you get an announcement, have a look at the geometry.

One of the issues is sharp angle intersections. The navigation sees it as a road "split" so it tells you which of the two ways you need to take.

But lets have a look at an example intersection i found within 30 seconds.

You see - the side road joining into the tertiary goes in a straight line into the main road. But is it really at this angle? Lets have a look at the aerial photo:

You can easily see that taken the rough direction of the road the intersection is correct. But are you actually merging like this into a busy main road? Typically not. You stop nearly at 90° to the main road to have clear sight into both directions. And when you take a very close look at the roads area you'll see a trumpet like widening of the road for the vehicles to stop at 90°.

So when following the center line of the road, and on the last 10m we follow the center line of the trumpet we suddenly see a 90° intersection show up.

See this example. The grey area shows your asymmetric trumpet like area, and the yellow line shows the real center line of your road.

So when drawing simple intersections like this, most likely in the countryside, have a closer look and try to maintain a half way 90° Angle. If you hear stray announcements. Remember the position and have a look at the geometry.