Roundabouts - why is a separate segment required?

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Roundabouts - why is a separate segment required?

Dave F
Hi
Could anyone give me an explanation for this line from https://wiki.openstreetmap.org/wiki/Tag:junction=roundabout

"Each road has to be connected with the roundabout in a separate node—that is, between these nodes a segment of the roundabout is required."

I see no requirement for a separate segment:
  • When a entering road shares a node with a roundabout then the router knows it's entered that roundabout by reading the tags on the circular way.
  • Whilst on that node, the router checks to see if there are any suitable exits. If there are, then it leaves the roundabout.
  • If not, it continues going around until it finds an appropriate exit.

Also, I'm more than a little annoyed at the inference it fails geometrically "because you did not trace the roundabout within its most external lane".

Cheers
DaveF



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Re: Roundabouts - why is a separate segment required?

Mateusz Konieczny-2
On Wed, 14 Feb 2018 14:53:07 +0000
Dave F <[hidden email]> wrote:

> I see no requirement for a separate segment:

Can you give example of roundabount (preferably, with good aerial
images) where such requirement makes mapping problematic?

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Re: Roundabouts - why is a separate segment required?

Maarten Deen
In reply to this post by Dave F
On 2018-02-14 15:53, Dave F wrote:

> Hi
> Could anyone give me an explanation for this line from
> https://wiki.openstreetmap.org/wiki/Tag:junction=roundabout
>
> "Each road has to be connected with the roundabout in a separate
> node—that is, between these nodes a segment of the roundabout is
> required."
>
> I see no requirement for a separate segment:
>
>   * When a entering road shares a node with a roundabout then the
> router knows it's entered that roundabout by reading the tags on the
> circular way.
>   * Whilst on that node, the router checks to see if there are any
> suitable exits. If there are, then it leaves the roundabout.
>   * If not, it continues going around until it finds an appropriate
> exit.

I'm not sure if you read the requirement right, but this tells mappers
not to connect the entry and exit road on the same node. If you were to
map it that way, the router will not see that you enter a roundabout and
need to exit at the first exit. It will just tell you to go right.
It is not (what I think you think) that there needs to be a separate way
between entrance and exit, the roundabout can be mapped as one way in
total.

Maarten

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Re: Roundabouts - why is a separate segment required?

Dave F
I think I have read it correctly.

https://www.openstreetmap.org/node/5408566797

It is easy to determine this shared node is part of the roundabout as
well as the entrance from Wapping & can exit along Commercial, or if
required, continue around the roundabout:
How is this different from, say, two side roads joining a main road at
the same node?,

Or even cross-roads. The router has to check to find out what road it's
crossing & find the appropriate exit, which, in the case of cross-roads,
will be on the same node.

DaveF

On 14/02/2018 16:17, Maarten Deen wrote:

> On 2018-02-14 15:53, Dave F wrote:
>> Hi
>> Could anyone give me an explanation for this line from
>> https://wiki.openstreetmap.org/wiki/Tag:junction=roundabout
>>
>> "Each road has to be connected with the roundabout in a separate
>> node—that is, between these nodes a segment of the roundabout is
>> required."
>>
>> I see no requirement for a separate segment:
>>
>>      * When a entering road shares a node with a roundabout then the
>> router knows it's entered that roundabout by reading the tags on the
>> circular way.
>>      * Whilst on that node, the router checks to see if there are any
>> suitable exits. If there are, then it leaves the roundabout.
>>      * If not, it continues going around until it finds an appropriate
>> exit.
>
> I'm not sure if you read the requirement right, but this tells mappers
> not to connect the entry and exit road on the same node. If you were
> to map it that way, the router will not see that you enter a
> roundabout and need to exit at the first exit. It will just tell you
> to go right.
> It is not (what I think you think) that there needs to be a separate
> way between entrance and exit, the roundabout can be mapped as one way
> in total.
>
> Maarten


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Re: Roundabouts - why is a separate segment required?

Colin Smale

Based on my experiences with mkgmap it's not so much a routing problem as a navigation problem. The router will pick the correct path through the graph but the translation to "human instructions" get confused, like the exit numbers and the way the roundabouts display. Turning right at a roundabout, i.e. taking the third exit, might show as straight on and the instructions may refer to the first exit.

 


On 2018-02-14 17:39, Dave F wrote:

I think I have read it correctly.

https://www.openstreetmap.org/node/5408566797

It is easy to determine this shared node is part of the roundabout as well as the entrance from Wapping & can exit along Commercial, or if required, continue around the roundabout:
How is this different from, say, two side roads joining a main road at the same node?,

Or even cross-roads. The router has to check to find out what road it's crossing & find the appropriate exit, which, in the case of cross-roads, will be on the same node.

DaveF

On 14/02/2018 16:17, Maarten Deen wrote:
On 2018-02-14 15:53, Dave F wrote:
Hi
Could anyone give me an explanation for this line from
https://wiki.openstreetmap.org/wiki/Tag:junction=roundabout

"Each road has to be connected with the roundabout in a separate
node—that is, between these nodes a segment of the roundabout is
required."

I see no requirement for a separate segment:

     * When a entering road shares a node with a roundabout then the
router knows it's entered that roundabout by reading the tags on the
circular way.
     * Whilst on that node, the router checks to see if there are any
suitable exits. If there are, then it leaves the roundabout.
     * If not, it continues going around until it finds an appropriate
exit.

I'm not sure if you read the requirement right, but this tells mappers not to connect the entry and exit road on the same node. If you were to map it that way, the router will not see that you enter a roundabout and need to exit at the first exit. It will just tell you to go right.
It is not (what I think you think) that there needs to be a separate way between entrance and exit, the roundabout can be mapped as one way in total.

Maarten


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Re: Roundabouts - why is a separate segment required?

Maarten Deen
In reply to this post by Dave F
On 2018-02-14 17:39, Dave F wrote:
> I think I have read it correctly.
>
> https://www.openstreetmap.org/node/5408566797
>
> It is easy to determine this shared node is part of the roundabout as
> well as the entrance from Wapping & can exit along Commercial, or if
> required, continue around the roundabout:
> How is this different from, say, two side roads joining a main road at
> the same node?,

Because a machine can not determine if you are actually entering the
roundabout or not. Technically speaking you are not because you are just
touching one node of the roundabout.

Suppose your example with the two side roads, if you were to go from
Side Road A to Side Road B via the node that is connected to Main Road
A, would you want the router to give you instructions to "enter Main
Road A and turn into Side Road B"? No, it gives you directions to "turn
into Side Road B". The same at this roundabout, going from Wapping Road
to Commercial Road it will tell you to "turn left into Commercial Road"
and not to "enter the roundabout and exit at the first exit into
Commercial Road".
At may be that the road layout is such that you don't enter the
roundabout, I don't know but when you do have to enter the roundabout,
you have to leave a segment between the entry and exit or the routing
instructions will be wrong.

Just connecting to a road on a node does not mean you enter that road.
The same at intersections, if you cross a road (connected by a node) you
do not enter that road so you do not need instructions for it.

Maarten

> On 14/02/2018 16:17, Maarten Deen wrote:
>> On 2018-02-14 15:53, Dave F wrote:
>>> Hi
>>> Could anyone give me an explanation for this line from
>>> https://wiki.openstreetmap.org/wiki/Tag:junction=roundabout
>>>
>>> "Each road has to be connected with the roundabout in a separate
>>> node—that is, between these nodes a segment of the roundabout is
>>> required."
>>>
>>> I see no requirement for a separate segment:
>>>
>>>      * When a entering road shares a node with a roundabout then the
>>> router knows it's entered that roundabout by reading the tags on the
>>> circular way.
>>>      * Whilst on that node, the router checks to see if there are any
>>> suitable exits. If there are, then it leaves the roundabout.
>>>      * If not, it continues going around until it finds an
>>> appropriate
>>> exit.
>>
>> I'm not sure if you read the requirement right, but this tells mappers
>> not to connect the entry and exit road on the same node. If you were
>> to map it that way, the router will not see that you enter a
>> roundabout and need to exit at the first exit. It will just tell you
>> to go right.
>> It is not (what I think you think) that there needs to be a separate
>> way between entrance and exit, the roundabout can be mapped as one way
>> in total.
>>
>> Maarten
>
>
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Re: Roundabouts - why is a separate segment required?

Dave F
In reply to this post by Colin Smale
On 14/02/2018 16:50, Colin Smale wrote:

Based on my experiences with mkgmap it's not so much a routing problem as a navigation problem. The router will pick the correct path through the graph but the translation to "human instructions" get confused, like the exit numbers and the way the roundabouts display. Turning right at a roundabout, i.e. taking the third exit, might show as straight on and the instructions may refer to the first exit.


This would only occur if there was no check to see if it's a roundabout first:

  • Enter
  • Check if roundabout
  • (While still on the same node) Start counting entrances/exits

I'm glad you mentioned mkgmap as I suspect this is where this mapping instruction originated. From previous conversations on their forum it's clear some try to fudge OSM as they lack the skill to program mkgmap correctly.

OSM contributors should not have to map incorrectly to suit these data users

DaveF.

 


On 2018-02-14 17:39, Dave F wrote:

I think I have read it correctly.

https://www.openstreetmap.org/node/5408566797

It is easy to determine this shared node is part of the roundabout as well as the entrance from Wapping & can exit along Commercial, or if required, continue around the roundabout:
How is this different from, say, two side roads joining a main road at the same node?,

Or even cross-roads. The router has to check to find out what road it's crossing & find the appropriate exit, which, in the case of cross-roads, will be on the same node.

DaveF

On 14/02/2018 16:17, Maarten Deen wrote:
On 2018-02-14 15:53, Dave F wrote:
Hi
Could anyone give me an explanation for this line from
https://wiki.openstreetmap.org/wiki/Tag:junction=roundabout

"Each road has to be connected with the roundabout in a separate
node—that is, between these nodes a segment of the roundabout is
required."

I see no requirement for a separate segment:

     * When a entering road shares a node with a roundabout then the
router knows it's entered that roundabout by reading the tags on the
circular way.
     * Whilst on that node, the router checks to see if there are any
suitable exits. If there are, then it leaves the roundabout.
     * If not, it continues going around until it finds an appropriate
exit.

I'm not sure if you read the requirement right, but this tells mappers not to connect the entry and exit road on the same node. If you were to map it that way, the router will not see that you enter a roundabout and need to exit at the first exit. It will just tell you to go right.
It is not (what I think you think) that there needs to be a separate way between entrance and exit, the roundabout can be mapped as one way in total.

Maarten


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Re: Roundabouts - why is a separate segment required?

Dave F
In reply to this post by Maarten Deen
On 14/02/2018 17:13, Maarten Deen wrote:

> On 2018-02-14 17:39, Dave F wrote:
>> I think I have read it correctly.
>>
>> https://www.openstreetmap.org/node/5408566797
>>
>> It is easy to determine this shared node is part of the roundabout as
>> well as the entrance from Wapping & can exit along Commercial, or if
>> required, continue around the roundabout:
>> How is this different from, say, two side roads joining a main road at
>> the same node?,
>
> Because a machine can not determine if you are actually entering the
> roundabout or not.

Yes it can. It has the junction=roundabout tag on the way.

> Technically speaking you are not because you are just touching one
> node of the roundabout.

Yes you are. You may not be on there very long, but you approach the
roundabout, pass the signs saying it's a roundabout, give way to those
already on it, you enter it & then indicate that you're leaving it.

>
> The same at this roundabout, going from Wapping Road to Commercial
> Road it will tell you to "turn left into Commercial Road" and not to
> "enter the roundabout and exit at the first exit into Commercial Road".

Noting my comment above, if a router doesn't tell you the latter, then
it's a poor program. Commercial Rd is the first exit

> Just connecting to a road on a node does not mean you enter that road.
> The same at intersections, if you cross a road (connected by a node)
> you do not enter that road so you do not need instructions for it.

A router has to be aware of it & know what it's attributes are, to
decide if it needs to go along it. It does this from a *single* node. If
it can do it at intersections it can do it on roundabouts.

Roundabouts are just another type of intersection.

DaveF



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Re: Roundabouts - why is a separate segment required?

Maarten Deen
On 2018-02-14 18:50, Dave F wrote:

> On 14/02/2018 17:13, Maarten Deen wrote:
>> On 2018-02-14 17:39, Dave F wrote:
>>> I think I have read it correctly.
>>>
>>> https://www.openstreetmap.org/node/5408566797
>>>
>>> It is easy to determine this shared node is part of the roundabout as
>>> well as the entrance from Wapping & can exit along Commercial, or if
>>> required, continue around the roundabout:
>>> How is this different from, say, two side roads joining a main road
>>> at
>>> the same node?,
>>
>> Because a machine can not determine if you are actually entering the
>> roundabout or not.
>
> Yes it can. It has the junction=roundabout tag on the way.
>
>> Technically speaking you are not because you are just touching one
>> node of the roundabout.
>
> Yes you are. You may not be on there very long, but you approach the
> roundabout, pass the signs saying it's a roundabout, give way to those
> already on it, you enter it & then indicate that you're leaving it.

Not from a data standpoint.

>> Just connecting to a road on a node does not mean you enter that road.
>> The same at intersections, if you cross a road (connected by a node)
>> you do not enter that road so you do not need instructions for it.
>
> A router has to be aware of it & know what it's attributes are, to
> decide if it needs to go along it. It does this from a *single* node.

It doesn't work like that anywhere in OSM. I can cross a road that I'm
not allowed to drive on. The router does not need to know anything about
the road that I'm crossing and I can always cross a road that I'm not
allowed to enter.
It would make mapping extremely awkward if that were not so.

Maarten

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Re: Roundabouts - why is a separate segment required?

Johan C
In reply to this post by Dave F


2018-02-14 18:50 GMT+01:00 Dave F <[hidden email]>:
On 14/02/2018 17:13, Maarten Deen wrote:
On 2018-02-14 17:39, Dave F wrote:
I think I have read it correctly.

https://www.openstreetmap.org/node/5408566797

It is easy to determine this shared node is part of the roundabout as
well as the entrance from Wapping & can exit along Commercial, or if
required, continue around the roundabout:
How is this different from, say, two side roads joining a main road at
the same node?,

Because a machine can not determine if you are actually entering the roundabout or not.

Yes it can. It has the junction=roundabout tag on the way.

Technically speaking you are not because you are just touching one node of the roundabout.

Yes you are. You may not be on there very long, but you approach the roundabout, pass the signs saying it's a roundabout, give way to those already on it, you enter it & then indicate that you're leaving it.


The same at this roundabout, going from Wapping Road to Commercial Road it will tell you to "turn left into Commercial Road" and not to "enter the roundabout and exit at the first exit into Commercial Road".

Noting my comment above, if a router doesn't tell you the latter, then it's a poor program. Commercial Rd is the first exit

Just connecting to a road on a node does not mean you enter that road. The same at intersections, if you cross a road (connected by a node) you do not enter that road so you do not need instructions for it.

A router has to be aware of it & know what it's attributes are, to decide if it needs to go along it. It does this from a *single* node. If it can do it at intersections it can do it on roundabouts.

Roundabouts are just another type of intersection.


No, they are not. Roundabouts are special types of intersections. They have a way on which you can drive round. And round. And round. And they have other ways leading to and from this round way. Whenever you enter the roundabout you drive on this round way, even if it's just for a metre. And then you exit this round way on to a different way. 

The present tagging (used since 2005 or so, and all around the globe) is fine.
 
DaveF




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Re: Roundabouts - why is a separate segment required?

Andy Townsend
In reply to this post by Maarten Deen
(... snip ...)
>>> Technically speaking you are not because you are just touching one
>>> node of the roundabout.
>>
>> Yes you are. You may not be on there very long, but you approach the
>> roundabout, pass the signs saying it's a roundabout, give way to those
>> already on it, you enter it & then indicate that you're leaving it.
>
> Not from a data standpoint.

OSM's "lines and points" abstraction is just an abstraction of the real
world.  In the real world you're on a road, and you're joining the
roundabout, staying there for a bit and then leaving it again on the
next road.  Having one exit node not joined to the next entry node
better represents the real-world situation*.

Best Regards,
Andy


* unless you happen to be riding a Spherical Cow along one of those
"frictionless surfaces" I remember from Applied Maths at school many
years ago.

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Re: Roundabouts - why is a separate segment required?

Dave F
In reply to this post by Johan C
On 14/02/2018 18:23, Johan C wrote:
No, they are not. Roundabouts are special types of intersections.
Which is another type of intersection.

They have a way on which you can drive round. And round. And round. And they have other ways leading to and from this round way. Whenever you enter the roundabout you drive on this round way, even if it's just for a metre. And then you exit this round way on to a different way. 

The present tagging (used since 2005 or so, and all around the globe) is fine.
 

To repeat myself. You can determine if you need to "drive on this round way" from a single node. No need for a section between entrance & exit.

DaveF
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Re: Roundabouts - why is a separate segment required?

Dave F
In reply to this post by Maarten Deen

> It doesn't work like that anywhere in OSM. I can cross a road that I'm
> not allowed to drive on. The router does not need to know anything
> about the road that I'm crossing and I can always cross a road that
> I'm not allowed to enter.
> It would make mapping extremely awkward if that were not so.

You're taking one example. Please don't extrapolate it to make it appear
it's every case.

Information about the way you're on or crossing can required for
numerous reasons. My original point, which seems to have got lost
somewhere, is you can determine where you are & where you need go from a
single node. Even on a roundabout.

DaveF



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Re: Roundabouts - why is a separate segment required?

Dave F
In reply to this post by Andy Townsend


On 14/02/2018 18:32, Andy Townsend wrote:
> Having one exit node not joined to the next entry node better
> represents the real-world situation*.

Disagree.
Sharing a node should make no difference to the real world or a router's
perception of it.

DaveF.



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Re: Roundabouts - why is a separate segment required?

Mark Wagner
In reply to this post by Dave F
On Wed, 14 Feb 2018 16:39:29 +0000
Dave F <[hidden email]> wrote:

> I think I have read it correctly.
>
> https://www.openstreetmap.org/node/5408566797
>
> It is easy to determine this shared node is part of the roundabout as
> well as the entrance from Wapping & can exit along Commercial, or if
> required, continue around the roundabout:
> How is this different from, say, two side roads joining a main road
> at the same node?,

If I'm judging the angles correctly, OsmAnd will not even announce that
intersection: the angle between Wapping and Commercial is shallow
enough that OsmAnd sees it as a single road, while the angle between
Wapping and the roundabout is sharp enough to not require a "keep
left" instruction.

In the general case, a router only needs to consider the ways that a
route actually passes over when creating directions.  By mapping a
roundabout entrance and exit sharing a single node, you've
introduced a special case: the router now needs to check all ways
connected to that node to see if any of them is part of a roundabout.

--
Mark

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Re: Roundabouts - why is a separate segment required?

Mateusz Konieczny-2
In reply to this post by Johan C
On Wed, 14 Feb 2018 19:23:39 +0100
Johan C <[hidden email]> wrote:

> The present tagging (used since 2005 or so, and all around the globe)
> is fine.

I agree, I see no problem with this recommendation.

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Re: Roundabouts - why is a separate segment required?

Matej Lieskovský
If two ways enter a roundabout at the same point, you can turn from
road A into road B instantly,
but going from B to A will require going around the entire roundabout.
For a router to detect this, it would have to check (for every
encountered node):
1) is this a part of a roundabout?
2) if yes, which of the turns I can make here would require going
around the roundabout?

Note that 1) is a query for all ways using that node and 2) requires
geometry data (instead of just network)

While this is doable (or could be solved with turn restrictions),
it makes roundabouts a much more of a special case.
The current tagging of a one-way circular way is much easier for
routing software to deal with
and is conceptually "cleaner".

Also, having the intersection directly on the roundabout
is potentially also a valid way of drawing a regular intersection
directly next to the roundabout.
(I would not be surprised to see such a crazy construction somewhere)

On 14 February 2018 at 23:04, Mateusz Konieczny <[hidden email]> wrote:
On Wed, 14 Feb 2018 19:23:39 +0100
Johan C <[hidden email]> wrote:

> The present tagging (used since 2005 or so, and all around the globe)
> is fine.

I agree, I see no problem with this recommendation.

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Re: Roundabouts - why is a separate segment required?

Tom Pfeifer
In reply to this post by Mark Wagner
On 14.02.2018 17:39, Dave F wrote:
> https://www.openstreetmap.org/node/5408566797

It appears that you already engage in an edit war, although half a dozen people here tell you, from
a variety of perspectives, that you are wrong.
https://www.openstreetmap.org/changeset/56352276

On 14.02.2018 18:27, Dave F wrote:
 > I'm glad you mentioned mkgmap as I suspect this is where this mapping instruction originated. From
 > previous conversations on their forum it's clear some try to fudge OSM as they lack the skill to
 > program mkgmap correctly.

It would help to maintain objectivity to leave out any rants about particular programmers of
particular software. I have seen other routing engines fail when the situation occurs that you have
created.

On 14.02.2018 18:27, Dave F wrote:
 > This would only occur if there was no check to see if it's a roundabout first:
 >   * Enter
 >   * Check if roundabout
 >   * (While still on the same node) Start counting entrances/exits

In which routing engine did you implement this? From which experience do you speak?
Or is it just pseudocode that fell off your sleeve without being tested in an implementation?

On 14.02.2018 21:44, Mark Wagner wrote:
> In the general case, a router only needs to consider the ways that a
> route actually passes over when creating directions.  By mapping a
> roundabout entrance and exit sharing a single node, you've
> introduced a special case: the router now needs to check all ways
> connected to that node to see if any of them is part of a roundabout.

Yes and to reiterate, to separate roundabouts from non-roundabouts, you would need to check that
special case not only at roundabouts, you would need to check its absence at _any_ node connecting
_any_ two road segments, even if these are not junctions. You would need to check if there starts a
roundabout segment or not, and if all segments of such roundabout loop back to the original node.

As the check you propose is against the basics of graph theory which is behind routing algorithms,
it would create an immense performance burden on the algorithms. That would make you mourn about the
skills of the programmers, again.

May we ask you to undo your revert in CS 56352276? You still have not explained how the two node
solution "fudge OSM".

tom

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Re: Roundabouts - why is a separate segment required?

Darafei "Komяpa" Praliaskouski
In reply to this post by Dave F

To repeat myself. You can determine if you need to "drive on this round way" from a single node. No need for a section between entrance & exit.

You can. You're at it now.
Someone else can be thinking of all the other cases while implementing their thing.

Sometimes people think "this is obvious, everyone will parse it that way", and then find out that someone just checks oneway tag for non-empty and their oneway=-1 are shown in reverse, and there's nothing you can do about it.

You can either make an ambiguous scheme that allows two different interpretations ("you enter roundabout the moment you touch the way" and "you did not enter roundabout and it's just a turn") and have difficulties counting exits. Or you can follow the suggestion, add a small segment and make the data that can be understood in the same way by consumers with different assumptions.

It's better to have compatible defaults.

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Re: Roundabouts - why is a separate segment required?

Mateusz Konieczny-2
In reply to this post by Tom Pfeifer
On Thu, 15 Feb 2018 09:52:19 +0100
Tom Pfeifer <[hidden email]> wrote:

> May we ask you to undo your revert in CS 56352276? You still have not
> explained how the two node solution "fudge OSM".

Especially, as from looking at aerial images it is clear that these
roads are not entering/leaving at the same point.

It would help other mappers (I ignore routing software here) to not map
different things using the same node.

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