[OSM-talk] (magical?) road detector

Thibault North tnorth at fedoraproject.org
Sun Feb 6 02:01:46 GMT 2011


A few remarks again below:

On February 5, 2011 06:57:42 pm Ido Omer wrote:
> Hi Elizabeth,
> 
> Like in any interesting problem, we knew we will not be able to come up
> with a general solution for all roads under any condition. We assume most
> people are mapping asphalt/concrete roads and we decided to focus our
> efforts there. 

And even in that case, good luck! :-)
See for example these two shots from Bing maps, Montreal, and how bad is the 
road extraction:
http://tnorth.ch/blog/public/Dijkstra/tests/montreal1_out.jpg
http://tnorth.ch/blog/public/Dijkstra/tests/montreal2_out.jpg
(Showing, by the way, that a shortest-path algorithm is not always what we 
want to extract data in a robust way...)

Image processing in that area is a pain: shadows, trees over the road, gray 
roofs that look pretty much like a road...

> Eve there our solution is far from being perfect, but we
> believe in most circumstances it has a value that is larger than zero.

I don't really know what to think about that. The tool will maybe never be 
efficient to handle most cases, but can perhaps sometimes help, and it is a 
sufficient reason to try and improve it.
Allowing the user to have some kind of control might help the detection to be 
done properly.

> Strictly speaking, the current version doesn't look for road patterns; the
> next version might do that and provide a more general solution. It will be
> useful for us to learn what the community needs are; I'll be happy if you
> could refer me to a few examples.

In the mapping process (with JOSM or such tool), following roads is not really 
a problem, especially when they are not too sinuous (and that's when the road 
detector works well...). It can be done in a few clicks. Maybe the tool should 
try and act differently (but that is more GUI/UI related), and we could imagine 
the following scenario:
- The user wants to map roads and selects a "road extraction" tool.
- He roughly follows a road, maintaining a click (as you would do to paint 
with a brush in image processing softwares)
- The algorithm knows the approximate path, and tries to fit exactly the center 
of the road.

Regards,
Thibault

> BTW, by not using the detector its usefulness should be around zero, it is
> hard for me to imagine how can it drop much lower than that (at least for
> the long run, after you wasted a couple of minutes and found out it does
> not serve your needs)
> 
> Thanks,
> Ido
> 
> -----Original Message-----
> From: Elizabeth Dodd [mailto:edodd at billiau.net]
> Sent: Saturday, February 05, 2011 2:27 PM
> To: talk at openstreetmap.org
> Subject: Re: [OSM-talk] (magical?) road detector
> 
> I've made one attempt only at tracing a dirt road in dry country with the
> detector. I found its usefulness less than zero, as the system told me
> that too many tiles were involved and quit. Zooming in is not always
> practical to spot these roads, where the pattern recognition is a very
> long straight feature on a photograph, and same colour as the surrounds
> for a dirt road in dry country, and a dark colour for a railway. Having
> spotted the road then it is easier to find in zoomed images when looking
> for curved bits through (dry) waterways. I went back to doing it by hand,
> so for me it had a uselessness of less than zero.
> 
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