[mkgmap-dev] Simplifying ways

Sat Nov 28 21:56:16 GMT 2009

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.

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