[mkgmap-dev] Making splitter and MultiPolygon code play together

Tue Feb 2 21:34:50 GMT 2010

Hi Apollinaris,

On Tue, Feb 02, 2010 at 02:35:26AM -0800, Apollinaris Schoell wrote:
> ideally splitter should keep the whole relation in each tile which contains
> nodes/ways of this relation.
> mkgmap can fail on incomplete relations

Currently, splitter seems to preserve the relation, but it may discard
relation members or nodes constituting ways in the relation.  I investigated
http://www.openstreetmap.org/browse/relation/311221
that mkgmap was complaining about.

In the tile file, the multipolygon is defined as at
http://www.openstreetmap.org/browse/relation/311221
but the Palosaari island
http://www.openstreetmap.org/browse/way/43829168
is not defined in the tile file, because it is too far from the tile
boundaries (tile:lat<62.226562, island:lat>62.274).

Because mkgmap ignores unresolvable relation member references, it will
treat the multipolygon as if it only had the lake in role=outer and no
other members.  The lake polygon is intact in the tile file:

<way id='4717407'>
<nd ref='30037150'/>
<nd ref='335160092'/>
...
<tag k='name' v='Palokkajärvi'/>
<tag k='natural' v='water'/>
</way>

This matches what I got when downloading and saving in JOSM the following
point at the coastline:
http://www.openstreetmap.org/?lat=62.2807084&lon=25.7284731&zoom=18

Last, I grepped the node ids from the tile file.  Only 46 of the 182 nodes
are defined in the tile file.  The northmost included point is this one:
<node id='30037138' lat='62.2692188' lon='25.7421118'/>

Interestingly, the entire lake falls outside the tile boundaries.  The
tile is 59.414063,19.116211 to 62.226562,31.596680
but the south tip of the lake is lat=62.2605.

I can suggest two solutions to this issue:

* splitter should flag and preserve all nodes that belong to ways that belong
to multipolygon relations (I would not care about route relations, for example)

* mkgmap should discard multipolygon relations that consist of only one way

Best regards,

	Marko

> 
> On Tue, Feb 2, 2010 at 1:28 AM, Marko Mäkelä <marko.makela at iki.fi> wrote:
> 
> > Hi WanMil,
> >
> > I am considering to download a bigger map extract and cutting it to
> > rectangular tiles with splitter, so that I can work around the issues
> > with Geofabrik boundaries.
> >
> > Do you have an idea how to fix the problems at splitter tile boundaries,
> > such as these when splitting at lat=62.226562:
> >
> > http://www.openstreetmap.org/browse/relation/302897
> > http://www.openstreetmap.org/browse/relation/306274
> > http://www.openstreetmap.org/browse/relation/311221
> >
> >        Marko
> > _______________________________________________
> > mkgmap-dev mailing list
> > mkgmap-dev at lists.mkgmap.org.uk
> > http://www.mkgmap.org.uk/mailman/listinfo/mkgmap-dev
> >

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