> If that's the case and lower resolutions are used for routing I am quite
> sure that the merge-lines filter (and some other filters too) are doing
> harm.
>
I agree.
If I got this right, the list of points of a MapRoad should always start and end with
a CoordNode (no matter what resolution) when it "arrives" in the LineAddFilter,
else routing may not work.
This is not always the case:
The DouglasPeuckerFilter may remove the last point in case the previous point
has the same coords and is also preserved. This can be fixed easily.

The merge-lines filter can simply be changed to merge only non-routable ways,
which also allows to use it for all resolutions.

I also tried to change the LineSplitterFilter so that it splits roads only at coordNodes,
but that did not help (routing was broken). Maybe the reason for this is somewhere in the
code that writes to the img file, but I don't know where to look at,
so I decided to disable line merging for roads which seems to work fine.

Attached is a patch that implements the changes to allow using
--merge-lines with --route

The compiled binary is here:
http://files.mkgmap.org.uk/download/118/mkgmap.jar

Gerd