[mkgmap-dev] Simplifying ways

Sat Nov 28 22:47:55 GMT 2009


Johann Gail wrote:
> Mark Burton schrieb:
>   
>> 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.
>>   
>>     
> 'v8 patch (simplify) is really missleading. I too was unsure, which 
> patch you meant.
>
>   
>> 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.
>>   
>>     
> Thanks for explaining that in detail. I am fully aware of the fact, that 
> there is a one to one linking between the routing data and the drawing 
> data. But to my understanding, this linking exists only in resolution 
> 24, not in lower resolutions.
> (Or is there really routing information with lower resolution?? I doubt 
> that strongly.)
>   
I actually think that one should even thing about simplifying routable 
ways at resolution=24. Especially those that directly originate from GPS 
tracks (that will be difficult to find out though), maybe only if nodes 
are closer than 20m over longer distances. So say 6 nodes in 60m okay, 
20 nodes over 200m simplify because it seems to originate directly from 
a track. Often people put nodes on curves very close to each other, I 
think one would have to experiment whether routing works better if those 
nodes are simplified (maybe only for routing, but not for display?).

I noticed that some Garmin maps display 1:1 to openstreetmap data, but 
on route calculation show up to 10% shorter distance. Maybe here only 
routing nodes are simplifed (straighter line).
>> 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.
>>   
>>     
> Yes, you're right. My patch tries only to merge draing data. The initial 
> intention was only to speedup drawing on the etrexes. I could not 
> imagine how to merge routing data. In my opinion it should be absolutely 
> not neccessary to 'merge' rounting data, as there should be no mergable 
> arcs existing. If such nodes would exist, this would mean, that there is 
> a arc connecting exactly two lines of same type. This would mean a node 
> in the middle of road. This should not exist or at least filtered out in 
> previous stages. (Ignoring the special case of splitted 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.
>>
>>   
>>     
> Seems you missed the important fact, that the merging takes place only 
> at resolutions <=23. As far as I understand the whole thing, line data 
> at lower resolutions is used only for displaying.
> Please correct me, if I'm wrong.
> In this case the merging would have to take place before generating of 
> routing tables. Another coder has tried it (can't find the thread at the 
> moment), but this is also unfinished work.
>
> Regards,
> Johann
>
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>   
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