[mkgmap-dev] Support for "through_route" relations?

Wed Apr 10 12:14:02 BST 2013

On 2013-04-03 21:43, WanMil wrote:
>> 0>  In article<515C8E7F.4000004 at web.de>,
>> 0>  WanMil<URL:mailto:wmgcnfg at web.de>  ("Wanmil") wrote:
>>
>> Wanmil>  mkgmap contains some code to support "through_route" relations.
>> [...]
>> Wanmil>  Shall we still support this type of relation?
>> Wanmil>  Can anybody explain what this relation does?
Sorry - I've not kept my attention on this mailing list much recently, 
and I missed this one. I believe I may have been the user who asked 
Steve Ratcliffe for such a feature a few years back, and he agreed, and 
patched it in over a weekend or so.

Sorry it didn't get documented. I think it was initially nothing more 
than an experiment, but it works brilliantly for me (since I know what 
it does!) and evidently a few others must have picked up on it too.

Basically, it's markup that exists to convince the Garmin units to issue 
better verbal instructions when a road hits a junction but that the 
logical "main" route through that junction takes you to a different road.

I.e. the road-number (even the road class) changes magically in the 
junction, but on the ground you think you're still "on the main road" so 
to speak.

Look at 
http://www.openstreetmap.org/?lat=51.44852&lon=-3.49448&zoom=16&layers=M

(I'm going to have to fix this example: someone's broken it! The road 
heading NE towards Cowbridge is the B4270, not the B4268). But anyway - 
if you approach that junction from the south (on the B4270) intending to 
drive through Llysworney (further to the north) but without "through 
route" being enabled, your Garmin will tell you to "turn left" at that 
junction. However, if you look at it on the road, you actually want to 
"go straight on". There *is* a turn left, but that's down a small 
uncategorised road which you certainly don't want to be on!

Conversely, on the same junction, had you approached from the south (on 
the B4270) intending to drive to Cowbridge then it would say nothing as 
you hit the junction because after all you want to leave the junction on 
the B4270 - the same road that you approached on. But when you get 
there, to stay on the B4270 to Cowbridge is clearly a "turn right", and 
should be announced as such.

With the "through route" system it all works fine. Without it: confusing 
directions for the motorist. I request that the feature be kept - 
possibly enhanced. I have noticed that the existing system can be 
accidentally broken when others alter the roads around a junction and 
leave "through route" set on too many ways in the vicinity. I think 
Steve R's implementation only works if there are exactly 3 nodes in the 
relation: two roads and the junction node itself.

I did catch a few cases around my home town where some bus routes had 
been edited in, and when approach-roads to junctions marked with 
"through route" got split to allow for these bus routes doing funny 
things, then the two split fragments all ended up in the "through route" 
relation (taking the basic item-count above 3) and it failed. The system 
needs to count only the junction node and the two ways that must be 
*directly* connected to it.
>>
>> I think the best explanation is in the patch announcement email[1] -
>> perhaps that could be adapted for the OSM Wiki?
>>
>> [1]<URL: http://www.mail-archive.com/mkgmap-dev@lists.mkgmap.org.uk/msg04975.html>
> Thanks for the link!
>
> Now there have been 3 years after Mark proposed to document it in the
> Wiki... I will wait for another two months. If the relation is not
> documented after that time I think we can remove the code because the
> usage is very rare. I guess only a very few people know how to use this
> relation type correctly.
>   From my point of view we shouldn't support inofficial undocumented
> stuff (although it sounds useful...)
>
> Please don't feel offended. Just add the required documentation ;-)
>
> WanMil
>
> _______________________________________________

I could do the documentation (based on the text I just wrote above). But 
it would be nice to get that "split approach road" bug out of the system 
as currently implemented (sorry, Steve - more work for you I guess).

"Through route" should be used by more mappers! Definitely very useful 
on British roads - and I'd have thought useful on everyone else's too.

Steve Hosgood