[OSM-talk] About large streets

[Andy Robinson]

I'm with you on this Simon. If a line segment has no tags for direction then
it is assumed that you can travel from node A to B or from B to A. On a
motorway for example the segment for each travel direction (drawn as
separate linear features) simply has a tag indicating the direction A to B
is NOT permissible or B to A is NOT permissible. I prefer to opt out rather
than opt in so in my modelling of features you tag the restrictor attributes
rather than the acceptor attributes.

Andy

Andy Robinson
Andy_J_Robinson at blueyonder.co.uk 

>-----Original Message-----
>From: talk-bounces at openstreetmap.org [mailto:talk-
>bounces at openstreetmap.org] On Behalf Of Simon Hewison
>Sent: 23 February 2006 09:30
>To: talk at openstreetmap.org
>Subject: Re: [OSM-talk] About large streets
>
>Joerg Ostertag wrote:
>> This way you'd have to add two tracks for all normal 2-way streets, but
>you
>
>Thinking back to my dim and distant past, when I write a route planning
>program on a Commodore 64, I modeled each node to have a number of links
>to surrounding junctions (I didn't bother with intermediate points
>curves, but stored the distance in the node). I did have to remember to
>specify both directions for every link, but it did work, and it never
>went the wrong way down a one-way street. However, it still couldn't
>deal with no-right-turn and no-left-turn.
>
>My algorithm then wasn't brilliant, it consisted of first of all heading
>in the right direction, and finding any path it could (it had problems
>with rivers, and ended up backing up and starting from the previous
>node).. Once it had found a route, it proceeded to attempt to find
>better routes between portions of the route it had found, in an
>iterative manner. (It was a variant of Dijkstra's algorithm, but I
>hadn't heard of Dijkstra at the time)
>
>That was in the days of a 1MHz cpu, and relatively little RAM. These
>days, most PC-based route planning software seems to do things
>differently. I found a useful link.
>
>http://ai-depot.com/BotNavigation/Path.html
>
>So, when once of us (possibly me) starts a route-planning bit of
>software using OSM data, it would first of all need to get a data
>snapshot of an bounding box a little bigger than the start and end
>points, and then build it's own data structure, knowing that if there's
>a segment from A->B then unless there's tags otherwise, B->A is also
>possible. Therefore the OSM database doesn't itself need to store
>segments from A->B and B->A because that could be left to the route
>planning engine's own data structure.
>
>--
>Simon Hewison
>
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