[OSM-newbies] Overkill or Not?

[James Ewen]

On Sun, Oct 4, 2009 at 4:55 PM, Richard Weait <richard at weait.com> wrote:

> If someone had mapped that intersection as a simple divided-undivided
> it would be correct.  Adding the four flares would also be correct.
> We're building an abstraction of the real world, not a 1:1 exact
> replica; where would we put it?  And different mappers will disagree
> on exactly where my abstract model is too much or yours is not enough.

If you are creating a visual model, such as a map that would be
printed out on paper, you can get away with simplification of roadways
and intersections. If however the model you are creating could be used
by computers to create routes, then the model needs to be specific
enough to not allow the routing engine to try and send the user down
the wrong roads, or ask them to perform illegal turns or such.

> In either case using turn restrictions makes the abstract junction
> more accurate to routing programs.  You appear to be using
> way-placement to imply turn restrictions.  I have done that before,
> but only where physical barriers exist

Which is the case here. There are concrete barriers in place that
would make it very difficult to travel straight through the
intersection. There is a small island forcing traffic to the left, and
in the middle of the intersection, another small barrier that would
make it difficult to get out of the left hand turn, and continue
straight. The Yahoo imagery is not of high enough detail to be able to
make out the barriers, but you can see the rubber left on the road
surface where the traffic is forced to travel.

> I'm not saying that you are wrong, just that my choice of abstraction
> allows me to be lazier for the Cragavon intersection.  ;-)

I'm not wrong yet... the map is an import from the GeoBase data. It
has errors in it as far as I am concerned for this intersection, but I
have not yet figured out how to go about making it better.

> And that I should use turn restrictions for interesting junctions.  ;-)
> http://wiki.openstreetmap.org/wiki/Relation:restriction

I would consider using these turn restrictions for intersections where
the restrictions are simply legal restrictions imposed by
signage/bylaws. With physical barriers in place, it makes me want to
show a physical representation.

A turn restriction on a simple roadway intersection node would
probably work well for the routing engines, but leave some ambiguity
for a printed version of the map.

While everyone has different interpretations of how to depict the same
physical entity, the OSM project would really benefit from having some
type of uniformity. If the map is to be used by someone who is not
intimately familiar with OSM mapping, they have to try and figure out
how to interpret what they see on the map, and convert that to what
they will find on the ground. When the same type of intersection is
depicted 20 different ways in the OSM map, it makes if impossible for
the user to figure out what they will be coming upon next.

We should as a whole endeavor to try and come up with a fairly uniform
method of depicting ways found on the ground in the OSM database. I
spend a lot of time digging though the Wiki trying to make sure that
the tags I am using match up with that which I am trying to map. I
also rely on the collective knowledge of the users out there to help
me make the right choices.

I await your response now that I have clarified that this intersection
does indeed have physical barriers in place restricting traffic flow.

James
VE6SRV