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

Dave F davefoxfac63 at btinternet.com
Wed Feb 14 17:27:54 UTC 2018

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 

  * 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 

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


> 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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