Hi,
On 14.01.2011 13:30, Michał Borsuk wrote:
Even invariant lines become challenging for beginners, because the
concept of forward and backward roles is really difficult to grasp.
I may have got it wrong, but on a simple line from A to B, with
bus_stops serviced in both directions (a good majority of lines), I
don't see any use of the roles. I have the information that "here, at
this bus stops you have bus 105", and that's all I need now. I've used
roles on lines that have different paths, and with the scarse
information "out there", I managed to understand it, so again, we need
Actually, you made me just realize that by doing that (not adding
"forward" and "backard" to stops) I ignored the direction iformation,
which would be useful to the disabled, but indeed that's a lot of work.
It is not just useful, it is necessary in order to route pedestrians to
the correct one of two platforms.
By the way, the wiki page on route relations [1] is completely pointless
on this matter. From the section "Members":
<quote>
forward/backward
if a route should only be followed in one direction for some or all of
its length, the "role" can indicate this for some or all of the
constituent ways. "forward" means the route follows this way only in the
direction of the way and "backward" means the route runs only against
the direction of the way.
</quote>
OK, so if there is a route section served only in one direction, we can
use this to specify that direction.
<quote>
forward/backward:stop
A Bus stop or train halt, on the route, which is only be used in one
direction. The direction is related to the direction of the way, nothing
to do with towards/away from any bus station or terminus.
</quote>
Ouch... this obviously implies that stops be placed *on the way*, which
is wrong by itself (as for buses). Also, it is quite unlikely that a
stop is served in only one direction whereas the road it resides on is
used in both. For stops that are situated at a segment that is served
only in one direction, it is clear in what direction the stop is used,
isn't it.
Problem still not solved, we want to specify which direction a
particular platform/pole serves on a two-way segment. In order to do
that - assuming we are not using more than one relation - we must (1)
include the first and last stop as "first"/"last" members and (2) shift
the meaning of "forward_stop"/"backward_stop" to what it is just not
meant to be according to the description given and specify it for every
single element. (OK we could assume that, depending on the country,
platforms are right/left of the road, but I'm not really comfortable
with that.)
cheers
ant
[1] http://wiki.openstreetmap.org/wiki/Relation:route
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