Hi John Adding to what Thorsten has written: It can be impossible to reliably deduce what should be sea when just using the coastline data from within an OSM area extract. The extract area is expanded to be within a set of rectangular tiles and, as a result of this, coastline data might just stop within a tile; coastline is defined as having sea on one side and land on the other so having an "end" renders this ambiguous. mkgmap has various --generate-sea options to control what happens in this situation, but all can fail given particular circumstances. For this reason it is common for mkgmap map developers to use a world representation for sea and mkgmap has the option to use this rather than the extract coastline data (--precomp-sea=sea.zip) - see: https://www.mkgmap.org.uk/doc/options and https://www.mkgmap.org.uk/download/mkgmap.html Ticker On Fri, 2022-08-05 at 16:45 +0200, Thorsten Kukuk wrote:
Hi,
On Fri, Aug 05, john510185 wrote:
Hi, I hope someone can help with this. I'm a big fan of OpenMapChest and am amazed by Ben Konrath's lovely maps. There's a slight problem (of the first world type) in that in his map of Western Europe (specifically the Calais tile) the sea appears to have breached the sea wall and the land is inundated with blue! Ben says he can't help because it's due to a bad change someone has made to OSM. There's also something funny further west, around Le Havre. He says the problem will be solved by whoever made the change, but it's persisted now for quite a few iterations of his map.
Yes, the coastlines are very often broken in the OSM database, but for the last days I couldn't find any serious problem in that area. There are the usual wrong directions of islands, currently in the near of spain. Depending on how he builds his maps and handles the coastline, this could be a really big problem or no problem at all.
So it heavily depends on how he builds his maps.
There does appear to be a conflict between his British Isles map and the one for Europe. However, the problem isn't replicated in OsmAnd, or in any of the other Garmin-related maps I use, which would suggest that OSM itself is OK.
Your assumption is wrong. The coastline is most of the time broken in OSM, that's why people building maps have checks or use verified coastline data to avoid this flooding. E.g. by using pre-build sea data from here for garmin maps: https://www.thkukuk.de/osm/data/
Thorsten
Any wisdom greatly appreciated!
One of the replies I received was as follows:
Those maps are generated with mkgmap, and this sort of problem happens occasionally when coastlines get broken in OSM. You might look at the archives for mkgmap-dev@lists.mkgmap.org.uk or perhaps post there directly.
Any help with this would be much appreciated. I have MapSource screenshots available, but didn't attach them in case I messed anything up!
Cheers!
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