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[mkgmap-dev] Simplifying ways

From Felix Hartmann extremecarver at googlemail.com on Sat Nov 28 22:01:48 GMT 2009

Mark Burton wrote:
> Hi Felix (cc Johann)
>> Just as a follow up, I noticed that the v8 patch (simplify ...) - 
>> actually routing over long distances in Mapsource became worse. Some 
>> routes I can calculate without using this patch, don't calculate with 
>> the patch anymore.
> You posted this comment with a subject line related to "remove bogus
> nodes" but I guess you're really talking about the latest patch from
> Johann.
No Johanns (v8) patch actually improves long distance routing (2-3 
additional turns), while the remove bogus roads makes long distance 
routing worse for me (3-4 turns less). Sorry for the wrong text. I have 
no clue why the remove bogus nodes makes routing worse. v3 actually is 
great insofar as it allows several routable roads with different speed, 
but so far I have not been able to make such ways opposing oneways - but 
they are different speed. Currently playing around to find out how I can 
trick it into two opposing oneways (while the oneway is "artificial" in 
both directions in so far that I use incomplete rules to set it up).
> If you're saying that with that patch some long distance routing is
> broken it doesn't surprise me. I haven't looked at the patch in detail
> but my initial impression is that it can't work because it joins up
> line segments that "belong" to different roads. Here's my reasoning:
> Basically, each routable road generates 2 data items:
> 1 - the routing data (nodes and arcs used to calculate routes)
> 2 - the graphical line data (what you see on the screen)
> So if a route across the country uses, say, 4 different roads (for
> this discussion assume the roads just connect one end to the other
> without side roads) then there will be 4 arcs in the route. On the
> screen, the line segments for the 4 roads will be displayed. There is a
> one-to-one correspondence between the displayed lines and the
> underlying routing arcs. In fact, there is a real linkage between the
> line data and the routing info.
> The simplify patch in it's current form reduces the number of drawn
> lines by merging the graphical line data. It doesn't (unless I am
> mistaken) merge the routing information. So now there is no-longer a
> one-to-one correspondence between the displayed line(s) and the
> underlying routing arcs.
> Imagine that in our 4 road route, the 3 following roads are merged
> into the first. So now we have 1 graphical entity but we still have 4
> arcs. The linkage from the remaining line relates to the 1st arc only
> even though the graphical line spans the whole route.
> As I said, I haven't studied the patch in detail. I could be wrong.
> Cheers,
> Mark
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