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

From Mark Burton markb at ordern.com on Tue Dec 1 21:43:25 GMT 2009

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:

     |                   |
          |    |    |
          |    |    |

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. 



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