[OSM-talk] Cannot correct ring self intersection

[Jon Burgess]

On Thu, 2008-03-13 at 00:16 +0000, Jukka Rahkonen wrote:
> Hi,
> 
> I found about 100 topology errors from polygons in Finland with the following
> work flow:
> 
> - OSM-data are in PostGIS 
> - I took polygons with topology errors directly into OpenJUMP map with db query:
>   select * from osm_polygon where not  isvalid(way);
> - I used OpenJUMP topology check tool which creates a new point layer
>   showing the exact places of the errors
> - That layer I converted to gpx and loaded into JOSM. Then I downloaded OSM data

An efficient way to do it. I was going to write a script to do the
something similar but your approach sounds easier.

>   around error locations and made corrections.
> 
> Usually this works rather well.  But then I faced such a ring self-intersection
> that I cannot correct it.  It is about a lake that intersects itself but also
> shares one node with a road.  The place is here:  
> 
> http://www.openstreetmap.org/index.html?mlat=61.277669238738454&mlon=28.855010608222813&zoom=13
> 
> I was thinking that I should cut the lake polygon, draw a couple of new nodes
> and connect the lake together again, but I just cannot split the lake boundary.
>  How should I do it?

What I would do is:

Use the "shift-delete" to remove 2 of the 4 segements from the common
node. Then create another node some small distance away. Then add the
segments back again going via the new node.

Alternatively you could split the way into 2: select the way & the node
with the 4 segments, then tools->Split Way. I'm not certain whether this
will end up with 2 closed ways but I think it should.

	Jon