<html>
<body>
I'd say it depends how just is *just*, I've experimented, so here is a
sort of handbook:<br><br>
- 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).<br><br>
- 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.<br><br>
- 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.<br><br>
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.<br><br>
Mike<br>
Stockholm<br><br>
<br>
At 01:08 AM 9/7/2007, Dave Stubbs wrote:<br>
<blockquote type=cite class=cite cite="">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.<br><br>
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: <br><br>
All data 2007-09-05:<br>
<a href="http://dev.openstreetmap.org/%7Erandom/progress/london/london-070905-mapnik.png">
http://dev.openstreetmap.org/~random/progress/london/london-070905-mapnik.png
</a><br><br>
Just the mapped data: <br>
<a href="http://dev.openstreetmap.org/%7Erandom/progress/london-070905-mapped-only-mapnik.png">
http://dev.openstreetmap.org/~random/progress/london-070905-mapped-only-mapnik.png
</a><br><br>
The traced streets:<br>
<a href="http://dev.openstreetmap.org/%7Erandom/progress/london-070905-traced.png">
http://dev.openstreetmap.org/~random/progress/london-070905-traced.png</a>
<br><br>
<br>
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. <br><br>
Now my question is this: is this tracing actually useful?<br><br>
From my mapping perspective:<br>
- it makes it much harder to see what needs doing<br>
- 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. <br>
- 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 <br><br>
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 ;-) <br>
But maybe it fills some purpose I'm not aware of?<br>
I'm also interested about the people doing this... are you all just
incredibly bored, or is there a goal here?<br><br>
Any thoughts?<br><br>
Dave<br><br>
_______________________________________________<br>
talk mailing list<br>
talk@openstreetmap.org<br>
<a href="http://lists.openstreetmap.org/cgi-bin/mailman/listinfo/talk" eudora="autourl">
http://lists.openstreetmap.org/cgi-bin/mailman/listinfo/talk</a>
</blockquote></body>
</html>