[mkgmap-dev] [PATCH v2] - round coordinates when zoomed out

Sun Nov 22 19:58:24 GMT 2009


Johann Gail wrote:
> Hi,
>
> I'm the coder of the original dp code. Some comments to it:
>   
>>   
>>     
>>> BTW - Is it possible that there is a difference with the douglas peucker 
>>> filter whether a road is routable or not?
>>>     
>>>       
>> Yes, looking at the code, it seems that it doesn't change points that
>> are routing nodes so routable ways will be "less smooth" than
>> non-routable ways.
>>
>>   
>>     
> This is true.
> The reason for it laid in some possible inaccuracies at nodes.
> Take for example a T-crossing. This means two lines.A straigth (long) 
> one and one ending at the node. In my first attempt the DP algorithm 
> handles all nodes indifferent. But this means, that the long line could 
> be straighened out at this node, whereas the ending line will never move 
> its end point. So the end node laid not on the line. This error could 
> become visible at some resolutions.
>
> The only possibility to change this behaviour was to never move a node 
> connecting lines. This would affect all routing lines.
>   
>>> I think it is possible that non routable ways are smoothed heftier than 
>>> routable ways. This has nothing to do with this patch though, but the 
>>> difference only seems to become really visible on resolution <20
>>> (maybe resolution 24 is filtered if not routable, but not filtered if 
>>> routable. Then the more filtering is applied on each successive level, 
>>> the bigger the difference).
>>>     
>>>       
>> It think the DP filter does nothing at res 24 whether the way is
>> routable or not.
>>   
>>     
> Yes, as far as I can remember, there was some errors with the routing 
> tables, if I enable it at res 24. Also all other filters was disabled at 
> res 24. So I disabled it too.
>   
>>   
>>     
>>> Also I noticed that cgpsmapper maps, seem to have much higher smoothing 
>>> at lower resolutions compared to mkgmap (using 2.6). At resolution 24 
>>> and 22 it seem to be mostly identical though. Maybe we could also 
>>> increase the douglas peucker severity on resolution <20 compared to 
>>> right now. Even with very few lines mkgmap maps seem slow on GPS on 
>>> resolution 16 or 18.
>>>     
>>>       
>> That's worth experimenting with. 
>>
>> Perhaps Johann can comment on this, he's the original author of the DP
>> stuff.
>>
>>   
>>     
> Yes, at resolution 16 most of the nodes should be allowed to be 
> removable, as the connecting streets disappear at this level. But at the 
> time of writing I had no reasonable idea of how to get this information 
> inside the dp filter.
>   
I don't think that this really matters. One will only use level 18 or 16 
for orienation/map panning. Garmin City Navigator only goes down to 20, 
then it's already basemap - of course in Mapsource they have a nice 
filled overview map, which we are really missing. If we had a decent 
overview map, I would have 18 as last level, as on my GPS I never use 
level 17 or 16 cause it's too slow to be usable (even though it only has 
motorways and trunk roads, plus cities more than 1Mio inhabitants or 
Capitals.). Level 18 already could be much much more smoothed.

Have a look at the MalsingFreeMaps - I think their low resolutions are 
really well done (smoothing as well as level of detail). (though level 
22 and 20 is a bit too much information for my taste).
> Maybe in the meanwhile this information is available?
>
> Regards,
> Johann
>
>
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>   
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