OpenStreetMap

Auto roads, part 3

Posted by bdiscoe on 6 September 2013 in English (English)

In order to keep my road follower in the middle of the road, I tried switching from an incremental similarity (compare each point to the next) to absolute (compare each point to the starting point). Since the starting point is given in the middle of road, it happily follows the road center, until this happens: With incremental similarity, we were largely immune to disruptions along the side of the road, because we came upon them gradually. Now, a large shadow is sufficiently unlike our starting point that it scares the algorithm into swerving away from the shadow and running off the road. (I can sympathize with the algorithm. I did the same thing in a car once :-)

So, it just solves one problem, and exposes another.

I also tried the idea of, each step, taking a cross-section and look for symmetry to find where the "middle" of the road is. It didn't work; the RGB is just too noisy to find a clear center of symmetry.

Perhaps next I will try, like Richard said, a flood-fill. But rather than try to flood-fill the entire road network, just a local fill to find an approximate road extent. That might work, although there are plenty of examples where it definitely won't, like where the road is surrounded by similarly colored pixels: Here is an example of a road which my road follower has no trouble with (following, but not staying in the middle of). Attempting to flood-fill it is extremely sensitive to initial point; picking just the right point is OK but any other (or any looser tolerance) will fail in countless ways, including fill way off the road, or just getting one side of it (as in the image above), or filling everything except the middle.

Comment from mapryan on 6 September 2013 at 12:24

I would love to play with this if you need anyone to do some testing for you. I'm doing a lot of tracing round the SW of Ireland and the contrast if very high between the green fields and the tarmac so it might be an easier test bed

Comment from Mappo on 9 September 2013 at 13:16

Would it help the algorithm if the starting input where two points on a straight section of road (so all intermediate points would be valid input data too) rather than a single point? Perhaps even a series of straight line sections so the algorithm would only need to connect relatively short sections and would have a reasonable amount of human provided input to learn what the road looks like?

It seems like the algorithms and the editing user interface need to be designed together for maximum benefit. I keep thinking of the Photoshop-like smart select tools where they do an amazing amount of work for you but you still need to learn a few key combinations to get the most out of them by giving it the right starting data and then correcting it when it goes off course.

Comment from malenki on 30 November 2013 at 10:34

No updates since September regarding the tracing tool? sniff

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