[mkgmap-dev] Analysis of finnland mp seabounds processing

Sat Jan 30 08:05:48 GMT 2010

The generation of the sea polygons with 
--generate-sea=multipolygon,no-sea-sectors sometimes still produce 
flooded areas.

I have checked the finnland map and found the following things:

1. Some of the inner polygons generated for the seabounds mulitpolygon 
sometimes are inverted. Usually the inner polygons should denote all 
natural=land but some point out the sea are. This causes the flooding.

I cannot precisely point to lines of code where this happens. It might 
have to do with self intersecting polygons.

I have attached a GPX of one of self intersecting and inverted ones. 
Open JOSM, import the GPX, convert it to data layer, tag the polygon 
with natural=land.
- Compare it with the real map, you will see that it is inverted.
- Run the validator plugin. You will see that it has 27 duplicated nodes 
(I don't think that this is a problem). And there is an intersection in 
the east on the osmosis boundary shape. This should not create a real 
problem for mp. Maybe some parts of the polygon are dropped. But this 
might point to a problem of the sea multipolygon creation.

I could easily add a warning if a way intersects itself. Don't know if 
that's a performance issue but will test it.

2. Some islands seem to be tagged with natural=land. They are not 
considered in creating the sea multipolgyon.
Example: http://www.openstreetmap.org/browse/way/48666101

Hope these two things help to reduce the flooding.

WanMil

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