Re: [OSM-talk] Roundabouts - why is a separate segment required?

[Michael Andersen]

On torsdag den 15. februar 2018 17.20.08 CET Dave F wrote:
> On 14/02/2018 20:19, Matej Lieskovský wrote:
> > 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):
> But "going around the entire roundabout" is what always happens.
> 
> For clarity I'm reposting an example as many appear to misunderstand my PoV:
> 
> https://www.openstreetmap.org/node/5408566797
> 
> > 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?
> 
> These are tests that are performed even with segments between entrance &
> exit & so makes your points irrelevant to my point.
> 
> > The current tagging of a one-way circular way is much easier for
> > routing software to deal with
> > and is conceptually "cleaner".
> 
> Who said *anything* about changing tagging?
> 
> DaveF

Did anyone test to what extent your example actually works in practice?


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Re: [OSM-talk] Roundabouts - why is a separate segment required?

[Andy Townsend]

On 15/02/2018 16:49, Dave F wrote:
Again, the way containing the shared node has junction=roundabout in 
it. You are entering & exiting a roundabout


Using that argument elsewhere, if I drive northwest up the A446 at 
https://www.openstreetmap.org/way/1165#map=17/52.55098/-1.73102 "the way 
containing the shared node" (https://www.openstreetmap.org/node/25244878 
) is part of a motorway link.  That doesn't means I'm not allowed to 
cycle around the roundabout.


More generally, if you find yourself in a discussion and _everyone else_ 
disagrees with you, isn't that perhaps a bit of a hint that you might 
want to reconsider what you're saying?


Best Regards,

Andy


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Re: [OSM-talk] Roundabouts - why is a separate segment required?

[Dave F]

On 15/02/2018 08:52, Tom Pfeifer wrote:

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



The previous mapper move the roundabout to an *inaccurate* location in 
order "to fudge" a separating section. I legitimately reverted it so it 
was accurately located as per a survey I perform on the ground.




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?


If a router is unable to perform simple checks it's not worth its salt.


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, 


How does the 'roundabout' tag not  "separate"?

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".


No one's explained why the single node fails. It works fine at crossroads.


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Re: [OSM-talk] Roundabouts - why is a separate segment required?

[Dave F]

On 15/02/2018 09:11, Mateusz Konieczny wrote:


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


It's very poor mapping to assume aerial imagery is current.

DaveF

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Re: [OSM-talk] Roundabouts - why is a separate segment required?

[Dave F]

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.


If true I see that as a worrying error in OSMAnd. Entering form Wapping 
you pass roundabout signs approach give way markings for traffic on the 
roundabout. It's scary routers base their calculations on geometry 
rather than tags & node locations.



the router now needs to check all ways
connected to that node to see if any of them is part of a roundabout.


No different from when you encounter an exit, even if it's not attached 
to the same node as an entrance.


DaveF

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Re: [OSM-talk] Roundabouts - why is a separate segment required?

[Dave F]

On 14/02/2018 20:19, Matej Lieskovský wrote:

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):


But "going around the entire roundabout" is what always happens.

For clarity I'm reposting an example as many appear to misunderstand my PoV:

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


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?


These are tests that are performed even with segments between entrance & 
exit & so makes your points irrelevant to my point.



The current tagging of a one-way circular way is much easier for
routing software to deal with
and is conceptually "cleaner".


Who said *anything* about changing tagging?

DaveF
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Re: [OSM-talk] Roundabouts - why is a separate segment required?

[Dave F]

On 15/02/2018 10:05, Maarten Deen wrote:

On 2018-02-14 19:39, Dave F wrote:

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


You can not determine that from a single node. You need to load the 
whole way that connects to that node




But that what happens in all roundabout situations. You enter at a node 
& read the tags of the way which contains that node. Having an exit way 
attached to that node doesn't prevent this.


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

and than make a judgement call which roads connected to that node you 
will traverse (which you don't know, because from a topology 
standpoint you are not traversing that way).


In my example you can see you're on a roundabout & there's an exit. You 
can load both ways & calculate the correct direction to take, similar to 
a crossroads at a single node.




It is like Matej's example.
Suppose it is mapped with the entry and exit road connected to one 
node. Yes, you can see if there also connects a roundabout to that 
node and you can make the determination that in that case you need to 
traverse the roundabout in the correct direction.
But suppose there is not a roundabout connected but a (circular) way 
with a oneway direction. Then you also need to make the decision that 
you have to traverse that way.
But suppose the way is not circular (making you cross or touch a 
oneway street), than you can not do that.
When is a road circular? Most roads are circular from a topology 
standpoint, as in: you can reach a node on that way going in either 
direction. So you can not determine from a topology standpoint if a 
road is a circular road or a roundabout.


If what you say were to be true you would have problems navigating all 
types of junctions. Every junction is entered at a *node* all connected 
way's information is loaded by processing that node.


DaveF

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Re: [OSM-talk] Roundabouts - why is a separate segment required?

[Dave F]

On 15/02/2018 09:33, Andy Townsend wrote:

On 14/02/2018 18:57, Dave F wrote:



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. 


With separate nodes, you travel along the roundabout way for a small 
distance (as you do in real life).  With a shared node, you don't. 
They're topologically different.


Again, the way containing the shared node has junction=roundabout in it. 
You are entering & exiting a roundabout


DaveF

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Re: [OSM-talk] Roundabouts - why is a separate segment required?

[Colin Smale]

Just to throw another concept into the mix... so-called flare roads,
where a road joining a roundabout (or other junction for that matter)
splits into two short one-way segments which go either side of an
obstacle. Mkgmap tries to recognise them by seeing if they come together
within X metres. Why is this important? It's about how you present the
turn-off angle to the user. If you look at the pure geometric angles of
the OSM ways up to the next node, taking the first exit can easily get
rounded to "straight on". By looking at where the exit road is after
(say) 50m the you have a more accurate impression about whether it is a
right turn or whatever. Sharing nodes between the ingress from one road
and the egress to the next road makes this all more difficult.

As I said before, this is not about routing in a mathematical sense
(traversing the routing graphs) but about how the resulting list of
nodes/edges is presented to the user. It's great if you can seen the
calculated route superimposed on a base map, but if the written/spoken
instructions don't correspond to your perception as you approach the
junction, then the system has failed. 

On 2018-02-15 11:05, Maarten Deen wrote:

> On 2018-02-14 19:39, Dave F wrote: 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

You can not determine that from a single node. You need to load the
whole way that connects to that node and than make a judgement call
which roads connected to that node you will traverse (which you don't
know, because from a topology standpoint you are not traversing that
way).

It is like Matej's example.
Suppose it is mapped with the entry and exit road connected to one node.
Yes, you can see if there also connects a roundabout to that node and
you can make the determination that in that case you need to traverse
the roundabout in the correct direction.
But suppose there is not a roundabout connected but a (circular) way
with a oneway direction. Then you also need to make the decision that
you have to traverse that way.
But suppose the way is not circular (making you cross or touch a oneway
street), than you can not do that.
When is a road circular? Most roads are circular from a topology
standpoint, as in: you can reach a node on that way going in either
direction. So you can not determine from a topology standpoint if a road
is a circular road or a roundabout.

Your method of mapping makes for very elaborate edge cases and unwieldy
routing engines.

Regards,
Maarten

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Re: [OSM-talk] Roundabouts - why is a separate segment required?

[Maarten Deen]

On 2018-02-14 19:39, Dave F wrote:

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


You can not determine that from a single node. You need to load the 
whole way that connects to that node and than make a judgement call 
which roads connected to that node you will traverse (which you don't 
know, because from a topology standpoint you are not traversing that 
way).


It is like Matej's example.
Suppose it is mapped with the entry and exit road connected to one node. 
Yes, you can see if there also connects a roundabout to that node and 
you can make the determination that in that case you need to traverse 
the roundabout in the correct direction.
But suppose there is not a roundabout connected but a (circular) way 
with a oneway direction. Then you also need to make the decision that 
you have to traverse that way.
But suppose the way is not circular (making you cross or touch a oneway 
street), than you can not do that.
When is a road circular? Most roads are circular from a topology 
standpoint, as in: you can reach a node on that way going in either 
direction. So you can not determine from a topology standpoint if a road 
is a circular road or a roundabout.


Your method of mapping makes for very elaborate edge cases and unwieldy 
routing engines.


Regards,
Maarten




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Re: [OSM-talk] Roundabouts - why is a separate segment required?

[Andy Townsend]

On 14/02/2018 18:57, Dave F wrote:



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. 


With separate nodes, you travel along the roundabout way for a small 
distance (as you do in real life).  With a shared node, you don't. 
They're topologically different.


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Re: [OSM-talk] Roundabouts - why is a separate segment required?

[Mateusz Konieczny]

On Thu, 15 Feb 2018 09:52:19 +0100
Tom Pfeifer  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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Re: [OSM-talk] Roundabouts - why is a separate segment required?

[Komяpa]

>
>
> 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: [OSM-talk] Roundabouts - why is a separate segment required?

[Tom Pfeifer]

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: [OSM-talk] 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 
wrote:

> On Wed, 14 Feb 2018 19:23:39 +0100
> Johan C  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: [OSM-talk] Roundabouts - why is a separate segment required?

[Mateusz Konieczny]

On Wed, 14 Feb 2018 19:23:39 +0100
Johan C  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: [OSM-talk] Roundabouts - why is a separate segment required?

[Mark Wagner]

On Wed, 14 Feb 2018 16:39:29 +
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?,

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: [OSM-talk] Roundabouts - why is a separate segment required?

[Dave F]

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: [OSM-talk] Roundabouts - why is a separate segment required?

[Dave F]

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: [OSM-talk] Roundabouts - why is a separate segment required?

[Dave F]

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: [OSM-talk] Roundabouts - why is a separate segment required?

[Andy Townsend]

(... 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: [OSM-talk] Roundabouts - why is a separate segment required?

[Johan C]

2018-02-14 18:50 GMT+01:00 Dave F :

> 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: [OSM-talk] 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: [OSM-talk] Roundabouts - why is a separate segment required?

[Dave F]

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: [OSM-talk] Roundabouts - why is a separate segment required?

[Dave F]

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: [OSM-talk] Roundabouts - why is a separate segment required?

[Maarten Deen]

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: [OSM-talk] 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: [OSM-talk] 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: [OSM-talk] Roundabouts - why is a separate segment required?

[Maarten Deen]

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: [OSM-talk] Roundabouts - why is a separate segment required?

[Mateusz Konieczny]

On Wed, 14 Feb 2018 14:53:07 +
Dave F  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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[OSM-talk] 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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