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

Sun Oct 6 17:57:05 BST 2013

Hi WanMil,

I noticed that the branch had one more call to incHighwayCount().
I can't think of a situation where this causes problems, but
the call was correct.
Please merge again.

Gerd

WanMil wrote
> Hi Gerd,
> 
> great! I have merged the changes to the mergeroads branch and it runs 
> fine for me. I didn't yet check a style that uses continue statements 
> but your work seems to be *very* reasonable :-)
> 
> WanMil
> 
>> 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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