[OSM-talk] Is *just* tracing useful?

[Michael Collinson]

I'd say it depends how just is *just*, I've experimented, so here is 
a sort of handbook:

- Just tracing an area that I am unfamiliar with is of marginal use, 
the area looks mapped but street names are missing and intersections 
may be incorrect, for example what looks like a normal through way is 
actually foot/bike only for a short section.  Streets may also be too 
long (I've taken them up someone's driveway) or too short (a 
continuation is obscured by a tree or building).

- Just tracing an area that I am reasonably familiar with, even it 
was just one but freshly remembered visit, works extremely well. The 
gotchas above are avoided and in urban areas the resulting trace 
*shape* is actually much better than GPS.  Urban GPS tracing often 
cuts corners off or puts kinks in straight roads. The only caveat is 
whether there is any offset in the imagery used.

- Tracing an unfamiliar area and then jumping on the bike to collect 
the street names, take a few reference traverses and sort out the 
fiddly bits also works very well. I only need to ride 30%-50% of the 
distance I did previously.  It also increases the pedestrian/bike 
content of the map as I can detour as much as I like along short side 
foot/bike ways safe in the knowledge I have the actual street already 
mapped.  The trick is, when tracing, don't trace in anything you are 
unsure of.  Leave the less obvious junctions blank, i.e. don't join 
ways up.   That makes a second round of editting in either Potlatch 
or JOSM much, much easier.

I have, by the way, started religiously putting source=survey on 
anything I actually GPS trace or have at least visually checked 
out.  I find it very useful when going to edit the same area again.

Mike
Stockholm


At 01:08 AM 9/7/2007, Dave Stubbs wrote:
>I was out mapping today in London, and came across a few roads which 
>I thought were mapped, but actually it looks like they were just 
>traced off of the Yahoo imagery.
>
>I was interested as to just how much of London had actually been 
>mapped, and how much is simple tracing... so after applying a few 
>incredibly simple metrics to determine what's a traced road and 
>what's a mapped road, I came up with the following:
>
>All data 2007-09-05:
><http://dev.openstreetmap.org/%7Erandom/progress/london/london-070905-mapnik.png>http://dev.openstreetmap.org/~random/progress/london/london-070905-mapnik.png 
>
>
>Just the mapped data:
><http://dev.openstreetmap.org/%7Erandom/progress/london-070905-mapped-only-mapnik.png>http://dev.openstreetmap.org/~random/progress/london-070905-mapped-only-mapnik.png 
>
>
>The traced streets:
><http://dev.openstreetmap.org/%7Erandom/progress/london-070905-traced.png>http://dev.openstreetmap.org/~random/progress/london-070905-traced.png
>
>
>So there's a reasonable amount of traced stuff there. Also, some of 
>it has been there a while, so there doesn't seem to be any effort to 
>properly map it.
>
>Now my question is this: is this tracing actually useful?
>
> From my mapping perspective:
>  - it makes it much harder to see what needs doing
>  - when loaded onto my GPS the traced stuff becomes very difficult 
> to distinguish, so it takes me longer and I also miss stuff because 
> I assume it's been done.
>  - entering the data takes longer because I have to fix all the 
> oneway streets, and the ways that don't follow the roads, the ways 
> that aren't roads, and all the bits that were missed anyway... it's 
> much quicker to generate from scratch correctly
>
>In other words, I'd rather people didn't do it! In fact, I'd love it 
>if anyone tracing off of the imagery had to sign a legally binding 
>declaration saying they'd actually been to the place first ;-)
>But maybe it fills some purpose I'm not aware of?
>I'm also interested about the people doing this... are you all just 
>incredibly bored, or is there a goal here?
>
>Any thoughts?
>
>Dave
>
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