[OSM-talk] Topology (was: OSM the mediocre alternative)

[Artem Pavlenko]

Hi Frederik,

Have you got database schema and/or sample data for the model in  
'towards-a-new-...' ?

Cheers,
Artem

On 23 Apr 2007, at 13:11, Frederik Ramm wrote:

> Hi,
>
>> I believe that topology for editing is best implemented in the
>> *clients*,
>> rather than the server.
>
> Can't say I believe in that. I am not quite sure if I understand your
> points correctly so sorry if the following is stating the obvious.
>
> In my eyes, data without topology is as people wrote paragraphs of
> text in pre-wordprocessor days: It was just letters put on lines, and
> there were carriage returns, and some of these were just there
> because the line got to long, and others were there because a new
> paragraph started.
>
> Such data does not contain information about paragraphs. Clients
> working with such data can try to infer that information somehow
> using heuristics, but it is never going to be 100%.
>
>>  * clients are topological. That is, they download data from the API,
>>    and that data is such that it is a topological mesh. By dragging
>>    points onto one another, they 'snap' into place: snapping being
>> done
>>    on the client side.
>
> And then uploading it to the server as two points which just happen
> to be at the same location? To me there's a world of difference
> between two nodes that just happen to be at the same position and two
> features connecting at the same node because it is (topo)logically
> the same node.
>
> For example, there's a railway line going over a bridge, and below it
> is a river. The bridge is slightly curved, as is the river, prompting
> me to insert a node in the river and one in the railway line to
> "draw" them properly. I'd curse any editor that makes these two nodes
> "snap" into place! (Totally different situation than with a level
> crossing where such behaviour might be desirable.)
>
> I can think of a million other examples where level (or z-axis if you
> will) issues make it highly undesirable for anything to snap onto
> something else, like roads running under another road for a stretch,
> or even tunnels.
>
>> (Note that although OSM is topological, it doesn't enforce this
>> topology in any way... even JOSMlint doesn't seem to complain.)
>
> OSM enforces next to nothing, which is often a bad thing and leads
> people to do stupid things. Sometimes it is a good thing that gives
> people the chance to implement a clever idea though.
>
> Concerning topology, at least in the JOSM editor it works exactly
> like I want it - if I, the almighty mapper, decide that the node in
> the railway and the node in the road be different nodes, then I make
> them different (albeit, perhaps, sharing the same point in 2D space),
> and if I want them to be the same node, I make it so.
>
> Maplint will complain if there are several nodes at the exact same
> position, but this does not necessarily have to be an error.
>
>> Perhaps the answer is 'it is too hard for clients to create topology
>> from simple features' -- but why is that?
>
> It is always hard to create something from nothing. If the
> information isn't there, where should the editor get it from? Trying
> to infer topology from simple features, in my eyes, is pure guesswork
> and I will always be able to provide an example where it fails,
> blatantly.
>
> Bye
> Frederik
>
> -- 
> Frederik Ramm  ##  eMail frederik at remote.org  ##  N49°00.09' E008° 
> 23.33'
>
>
>
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Artem Pavlenko
http://mapnik.org



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