[mkgmap-dev] short arcs demystified, please try value 0

Sat Oct 5 10:56:06 BST 2013

Hi all,

I think I understand now most of the miracles related to 
short arcs and I am pretty sure that we can solve 
the known problems without adding source complexity.

1) In 2009 Mark Burton added the
remove-short-arcs option with r1049.
2) With r1060 he added the option to pass a parameter 
giving the minimum arc length to fix problems like 
"disappearing ways". Unfortunately, he missed the
real reason for the error, which IMHO was the
buggy implementation of --link-pois-to-ways
and --remove-short-arcs regarding the 
highway count causing more or less 
unpredictable results.
3) For four years people wondered what value to use
as a change of the value was likely to
"fix" a known routing error while also adding
other "not yet known" errors. 

Today I've committed r2736.
I my eyes (presuming all coding errors are fixed now) 
the only usable value is --remove-short-arcs=0, 
everything else is likely to 
create problems like distorted lines, wrong
routing (cars routed through footways)
and wrong exit hints in roundabouts.
The only advantage that I see is a slightly smaller
number of nodes, but this will soon be fixed
when the mergeroads branch is merged to trunk,
but I might have missed something.

So, in short:
All those that are using mkgmap to create routable
maps, please try r2736 with --remove-short-arcs=0
(binary is here http://files.mkgmap.org.uk/download/152/mkgmap.jar)
or use the mergeroads branch (requires style changes)
If you find any problem regarding routing, please 
let us know and try to provide details.

If I hear no complains until Oct 15, I like to change the default
back to 0 and print a warning if a different value is used.
If --remove-short-arcs=0 works fine for everybody, we
should remove the option.

Gerd

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