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[mkgmap-dev] Commit: r1410: Add --merge-lines option to combine lines at lower zoom levels.

From Felix Hartmann extremecarver at googlemail.com on Tue Dec 1 22:01:29 GMT 2009

On 01.12.2009 22:43, Mark Burton wrote:
> Hi Johann,
>> Hi Mark,
>> if your guess is true (it indeed seems so), then this would mean that
>> there is some linking between the graphical line and the arc. I know
>> there is a one to one link at highest resolution. There seems to be link
>> at lower resolutions too.
>> This may well be the unknown reason for the improving of the routing
>> with the merge-lines option.
> Perhaps, but that is a mystery to me.
>> What I do not understand: How can I link a long line to a lot of arcs.
>> There must be a way, as the original garmin maps contains long lines
>> too. Are there some arcs for lower resolutions too?
> So far, I have not seen any evidence of that.
> What I have seen in some maps created by cgpsmapper is what I call
> "short-circuit arcs". These are extra arcs that jump over some of the
> points in the way to make a more direct route. Here's some ascii art
> that illustrates what I mean:
>       ---------SCA---------
>       |                   |
> -----A++++B++++C++++D++++E----
>            |    |    |
>            |    |    |

Well I suppose those short-circuit lines are related to avoiding turn 
time penalties, or artificially set them. This is in my part (well said 
it often enough) - the most important thing for good routing, the only 
workaround being what I wrote down in the patch before - but of course 
this adds to map size much more than short-circuits would do.
Are these arcs visible within gpsmapedit? I have not seen them (maybe 
did not look well enough).
> The + signs show a single road that has nodes A-E. Nodes B, C and D
> have other roads connected to them. A and E are connected to other ways
> but they also have a short-circuit arc (SCA) between them. I have seen
> short-circuit arcs span from one end of a road to the other and also
> from an inner node (like B, C or D) to one of the ends. Note that the
> short-circuit arcs don't span roads, they just connect nodes within
> single roads.
> I have been planning to do some experiments to get to understand this
> more but have not found the time to do so yet.
> Cheers,
> Mark
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