Lake islands flooding


I am building map of Finland … a lot of lake alot of islands :wink:
But even quite big islands are flooded by lake water on my builded map.
I tried use costline file but it doesn’t help in this case.

Any suggestion for fixing this?


Do the maps from have the same problem? If yes, then I can’t help you but point you to the Mkgmap guru from Finland: Marko Mäkelä: marko [dot] makela [at] iki [dot] fi

(hope he doesn’t mind…)


to avoid flooding by errorneous coastline data I recommend to use precompiled sea data.
This compiles much quicker than the previous methods and the results regarding flooding problems are much better.
You can enable it using the parameter

precomp-sea=<directory with precompiled sea tiles>

instead of generate-sea.
You can download the sea tile from

In case that doesn’t help please check the multipolygons of the flooded lakes.


WanMil, I can remove this option from my script and replace with the precomp-sea option?


I just requested map from I’ll let you know when I got it.

I will contact him later if needed. Thank you


Thank you ,
just trying this method.I downloaded and extracted your file.
Now I have sea directory full of *osm.pbf files plus index.txt.gz file. But I getting this error with mkgmap 2179:
Could not open file: precomp-sea=/media/MEGA/MAPING/sea/

What I am doing wrong?


Your mkgmap version is too old to use precompiled sea tiles. It is possible since mkgmap 2278.

Do you use an option file or do you define the mkgmap options on command line? If you are using the command line please don’t forget the ‘–’ before the option (–precomp-sea=/media/MEGA/MAPING/sea)



For details please read the mkgmap help. It is possible to still use some generate-sea options (multipolygon, polygon and landtag) when setting the precomp-sea option. But for your generate-sea options it is sufficient to replace the generate-sea option with the new precomp-sea option.

Enjoy it!

By the way: the download contains PBF files for the sea tiles. But it is also possible to generate osm xml files as tiles which are editable with JOSM. So if you have a flooding problem anywhere in the world you can correct this in the local precompiled sea tiles. If you are interested in compiling the sea tiles yourself please read as a first introduction.


Much thanks for your work WanMil, I can use all the speed and convenience available :wink:

A few more Q’s:
I assume a precompiled sea file will become increasingly outdated over time when the coastline is edited?
Are new releases of fixed precompiled sea files planned?

You will enjoy that because the floodblocker took quite a lot of memory and CPU.

Yes of course. The sea tiles will outdate over time. But using the readme it should be possible to compile them yourself. I will also update them from time to time (but I also depend on updates at

Thanks again for learning us this new method.
It works great with newest mkgmap, building time seems to be much shorter and coastline looks great to me.

But unfortunatelly it doesn’t helped with flooded islands on lakes.
Can you advise me how to check these multipolygons? I never did it before.


You can use the multipolygon check of OSM inspector: Zoom in to the problematic lake and see if it shows an error.
If you are unsure what you have to correct please read the multipolygon page of the wiki first and/or ask Marko. Probably he knows what has to be changed.


@miramikes If the flooding happens across tile borders, sometimes it can be solved by setting the overlap higher in your splitter parameters; overlap=2000 is default, so try overlap=5000 or more.

@Lambertus: for the OFM you still need to use generate-sea=land-tag=natural=background with the precompiled sea file

I just got map from … It show some problematic areas better for me.
You can see on Finland map soutern part of Pielinen lake (Eastern Finland) number of small islands … on my map they are flooded.

I also noticed one other defect which suffer my map and map from too …

In some areas lake is “white as ground” and islands are blue.
I am using qlandkarteqt program for checking builded map on my computer … you can also see there tiles of map created by splitter.
This strange “opposite behavior” changing to “normal behavior” just on the border of these tiles.
Eastern Finland lake Saimaa south from Joensuu.


I’ve tried set splitter overlap=6000 … it changed affected areas, I mean problem appears in different place.
But problem persists unfortunatelly.


I think its a splitter problem. With very large multipolygons the spllitter can’t see the whole relation anymore and errors are made.
I noticed this problem too with very large lakes in Switzerland/Italy: Lakes of Geneva, Como, Maggiore or big forest areas.
The only thing you could do is trying to edit the tile boundaries so that the lake is totally in one tile or by making the overlap size bigger (10000).
A very large overlap creates other problems when the tiles are small (Paris area for instance).

I see Mkgmap reporting tons of multipolygon problems for each map update, so this could certainly be a cause.

Thanks. I’ve made the changes so we’ll see the result in the next update.

overlap size 10000 plus some polygon fixes made by Marko Makela fixed this problem.

Thanks to all involved!