[talk-au] Roundabouts and routing

[Ross Scanlon]

On Sat, 19 Dec 2009 07:31:59 +1000
Roy Wallace <waldo000000 at gmail.com> wrote:

> This is an interesting example. The "_messy" and "_simple" techniques,
> of course, could just as well be described as "_complete" and
> "_approximate", etc.

No _messy is over mapped and _simple is accurate.

> The real issue here is "what are we mapping" - and with the
> intersection example, the issue seems to be whether the ways should
> accurately correspond to geographic reality  (_messy), or not
> (_simple).

Messy is not geographic reality the turning lanes are not a separate way they are part of the main ways. The slip roads are separate ways so are included.
 
> Personally, I think they should (geographic reality is important in a
> map! :P), and hence I can see the appeal of _messy.

As above.
 
> However, Ross, if the _messy example really is "unusable", this is a
> problem. Ross, what exactly do you mean by "unusable"?

Try looking at this intersection on any gps screen (I use a garmin 76CXs) and it is basically unreadable there is too much there. The names are incorrect.  There is no road name "to Christine Avenue", etc.

> Note also that any problem with _messy does not then infer that
> _simple is a good approach either - it's clearly an approximation,
> which is fine in the interim, but sooner or later mappers are going to
> want to add more detail, and they surely should have a way to do so
> without making intersections "unusable".

_simple is an approximation yes but it conveys everything that is need to define that intersection. Additional information like turning lanes etc needs to be included in relations etc or other tags.

eg.

When travelling west on Christine Avenue tagged from the start of the 4 lanes up to the intersection with Bermuda Street.

tag
lanes=4

relation
type=restriction
restriction=lane_must_turn_right
lanes=2
to=Bermuda Street
from=Christine Avenue
via={node_number}

for those that are left turn then it would be

restriction=lane_must_turn_left

the number would be for the turning lanes only, this could then be interpreted by routers, renderers, etc and they could then provide prompts, draw as appropriate or if required.

It provides the data but declutters the map and makes it much easier to read in what every form, the maps need to be visually uncluttered but the data needs to be added.

It may not be necessary to include the to, from and via fields but is probably best.

I have modified the intersection immediately to the west on Christine Avenue with this type of relation on the west bound carriageway as an example.

It even validates correctly in josm, not bad for something I just thought up in the last ten minutes :).

-- 
Cheers
Ross