[OSM-talk] Cannot correct ring self intersection

[Rahkonen Jukka]

Jon Burgess wrote:

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.

At least it can be used directly without programming.  It is flexible as well,
the same system works as well with "select * from osm_line where highway='prmary'".

>>   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.

I got it corrected by doing some shift-deleting and redrawing. Thanks to Jörg who 
helped by separating the highway for me.


> 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.

I have not succeeded at all in splitting closed ways with JOSM. It always says to 
me that I should select a node from the middle of the road.  It seems to think 
that any node in the closed ring is the starting one.  Other way to think 
is that any node in the ring is in the middle...

-Jukka-