[OSM-talk] Clarifying and representing road markings at junctions

[David Earl]

On 01/10/2009 11:47, Roy Wallace wrote:
> On Thu, Oct 1, 2009 at 8:05 PM, David Earl <david at frankieandshadow.com> wrote:
>> It shows visually which the "main" road is at the junction and is a good
>> model of the physical arrangement.
> 
> IMHO it does not *explicitly* show the "continuations of roads at the
> junction". And even if you do think it works "visually", that is not
> sufficient - there are many uses for OSM data.
> 
>>> b) it makes a single *way* continue through the intersection.
>> err, no. If the road has the same name around the corner it can do. If the
>> minor but straight on road has the same name it could be a continuous way,
>> but I would normally break it at that point, not least so the name of the
>> "minor" road is clear. But where ways break is of no significance - you have
>> to break ways at all sorts of places because of changes in the environment
>> like speed limits, starts of bridges etc.
> 
> Ah, so are you saying that, in Martin's attached image, the red way
> and the yellow way should/could meet at the junction? If so, then IMHO
> it is even *less* clear that, e.g. traveling from the red to the grey
> way is a left turn, whereas traveling from the red way to the yellow
> way is uninterrupted.....

No I was referring to the real examples I quoted.

>> I think anyone looking at it would understand the arrangement on the ground,
>> and it does model the situation as I see it.
> 
> Don't be fooled - people are not the only ones that "look" at OSM data.

I don't understand this at all. I am just mapping what I see on the 
ground. And please don't patronise, I'm well aware of the uses of OSM 
data and have contributed to many of them.

The main road goes round a corner (and may or may not share the same 
name). I represent the corner even though there may be a straight kerb 
line on one side, when curvature exists e.g. on the opposite kerb or in 
the white lining.

The slight curve before the side road branches off might possibly allow 
a bright routing algorithm to describe it more accurately. But I think 
there would be ambiguity here independent of any mapping, because 
"straight on" is somewhat ambiguous, especially when it's not a complete 
right angle turn as sometimes happens.

In English I think I'd want to be told "follow the road round to the 
left" or some such in these circumstances, not a simple "turn left". A T 
junction certainly wouldn't achieve that possibility without more 
information. An explicit tagging would. But in the absence of that, 
modelling what's on the ground goes some way.

David