[mkgmap-dev] Analysis of MP issues in Finland

Fri Jan 29 21:05:57 GMT 2010

Hi WanMil,

> This mp was changed today (Jan 29th 2010, 06:33). Before the change a 
> small polygon in the north of the mp touched the outer ring. So it 
> contains to your first category overlapping lines.

Thanks for figuring this out.  I was too lazy to check previous versions,
and the Geofabrik extract is too big for my visualization tools.

By the way, I examined one multipolygon near the border more closely.
In the split osm.gz file, it contained fewer members than the relation
at the browse URL, and it had not been changed recently.  I wondered if
it is a splitter bug, but the huge finland.osm.bz2 contained the same error.
So, Geofabrik's tool of choice (Osmosis?) is apparently dropping the members
that are outside its cutting area.

> > 2010/01/29 12:11:44 WARNING (MultiPolygonRelation): 63240001.osm.gz: Multipolygon http://www.openstreetmap.org/browse/relation/375636 contains intersected or overlapping ways
> The inner and outer tags are interchanged. Mp could autodetect this but 
> only if it disregards the inner and outer tags completely. This makes 
> problems with your next examples, because the wrong assigned inner 
> polygons would be autodetected as outer polygons.

Thank you, I should have loaded it in JOSM to see it.
I just swapped the roles.

> > Last but not least, here is an idea for quality control.
> > I came across two lakes whose islands were in another lake
> > (the change message hinted that the lake had been split).
> > There is no warning for that.  The errors must be coming
> > from missing boundary information in the Geofabrik extract.
> >
> > http://www.openstreetmap.org/browse/relation/303320
> > http://www.openstreetmap.org/browse/relation/303321
> 
> Yep. At the moment the mp code is quite unspecific with its warning 
> messages. "Multipolygon http://www.openstreetmap.org/browse/relation/xxx 
> contains intersected or overlapping ways" is the only message if 
> something is not correctly inner/outer tagged, or some ways are 
> overlapping or inner ways belong to a complete other mp. I will think 
> about how to improve this.

It would be great if we could print a message saying "outer polygon does
not contain one or more inner polygons; should some outer and inner roles
be swapped?"

By the way, should we complain if there appear to be multiple outer polygons?
I know that the outer polygon can consist of multiple role=outer lines
(such as long coastlines split to 500 nodes each), but can there legitimately
be multiple outer polygons?  Hmm yes, thinking about administrative borders,
such as Russia+Kaliningrad (Königsberg), Denmark+Greenland, or from the past
West Germany + West Berlin.

Best regards,

	Marko