[mkgmap-dev] Handling of Multipolygons in Tile Splitter

Sun Aug 14 21:13:36 BST 2011

Hi,

I ran into a problem with flooded areas around the river Rhine.
A europe.osm.pbf from geofabrik.de dated 2011-07-31 was used.
I extracted a contiguous area containing roughly Germany and France 
using osmosis.
This extract was spit up into 369 parts using tile splitter r179.
After they have been converted using mkgmap I recognized at least 3 
flooded areas
at the river Rhine on my Oregon 550.
The riverbank areas extended wide into the land.
The riverbank in that europe.osm was defined by a really large multipolygon.
(in the meantime someone else split up the gigantic multipolygon into a 
number of smaller multipolygons).
This large multipolygon consists of 320 ways (counted inner and outer) 
and extends over
over at least 4 of 5 tiles that were created by tile splitter.
I had a look at one of the tile-files an noticed, that tile splitter 
removed all the ways that were completely
outside the bounding box of the respective tile.
However the multipolygon contained all 320 ways as members, but most of 
them (and the corresponding nodes)
were not included in the tile-file.
Im not sure wether tile splitter did some processing in order to close 
the multipolygon along the bounding box,
but probably this ist not the case.
This incomplete information in the tile file probably makes it 
impossible for mkgmap
to correctly clip the multipolygon along the bounding box and possibly 
leads to the flooded areas.

As I already mentioned, this large multipolygon is no more present in 
the actual osm database.
But I probably will be able to provide some test data if required as I 
still have the original data
and can reproduce this behavior.

Regards
Wolfgang