Hi list I spent the last 12 hours debugging sea generation problems in Europe. After digging through a lot of mkgmap and splitter code, I believe I understand the source of the issues now. Since data is processed in tiles, the sea generator will often encounter coastlines clipped at the tile boundaries. There are heuristics in the code that should produce valid sea multipolygons for tiles with incomplete coastlines as well. However, the heuristics fail for the Geofabrik extracts of several European countries. Here is what I believe the problem to be: The sea generator correctly identifies incomplete coastlines. It then attempts to determine for each such coastline whether it was clipped by a tile boundary. It is this test that fails for some countries, leading to missing or inverted sea. The source of this issue can be traced back to the splitter. Here, tile boundaries are expanded to coarse multiples of Garmin map units. Tiles therefore become larger than originally requested. If map data for the entire expanded area is available, this is OK. But when dealing with country extracts, there may be no data available for some of the expanded regions. Here is an ASCII art example, a rectangular extract with a coastline inside, shown as a double line: +---------+ | | | | |=========| | | +---------+ If this data is fed to mkgmap directly, the sea generator will correctly determine that the coastline was clipped by the left and right tile boundaries. If the data is passed through the splitter first, even if no splitting into multiple tiles should be needed, the tile boundaries are rounded to the multiples of Garmin map units mentioned above. The tile may thus grow: +-----------+ | | | | | | | ========= | | | | | +-----------+ The problem is that now, the coastline no longer touches the tile boundaries. The mkgmap sea generator is confused by this and produces invalid or no sea at all. An obvious but incorrect solution is for the sea generator to check whether the coastline reaches the *original* tile boundary, not the rounded one. This would require the splitter to pass the original boundary to mkgmap along with each tile. The real issue with this solution would be that even if mkgmap correctly detects a clipped coastline, there is no valid data between the original and rounded boundaries. To construct a proper sea multipolygon, mkgmap would have to synthesize a coastline in that space, likely introducing ugly artifacts. A correct solution, already implemented in mkgmap today, is to read coastlines from a separate file. The file should contain coastlines that extend at least as far as the rounded tile boundaries. From the mail archives, it appears that WanMil has identified the same issue before and proposed this very solution [1]. Coastlines for a larger area can be extracted using: osmosis \ --rb larger_area.osm.pbf \ --tf accept-ways natural=coastline \ --tf reject-relations \ --used-node \ --wb coastlines.osm.pbf omitmetadata=true The coaslines.osm.pbf file can then be fed to mkgmap via the --coastlinefile option. While WanMil described this solution in December 2010 already, it seems to have been buried in the mailing list without becoming common knowledge. Another mailing list post [2] shows that the --coastlinefile had been broken for a while without anyone noticing, confirming that it is not in common use. After spending 12 hours on this today, I hope that others will benefit from this write-up instead of having to repeat my odyssey. Anyone wanting to use the --coastlinefile option will require coastlines for a larger region than the extract they are processing. While the option does allow several files to be specified whose contents are then concatenated, it is easiest to load a single file with coastlines for a larger area. I have prepared two such files, one from today's Geofabrik europe.osm.pbf extract, the other from the most recent planet-latest.osm.pbf. Both files can be found at [3]. I am providing these files so that anyone working with extracts can download compact coastlines only instead of having to extract them from the huge europe.osm.pbf or planet-latest.osm.pbf files. Right now, the files were generated once-off. If they are found to be useful by others, I am happy to automate the process and make updated coastline files available on a regular basis. Finally, a word of warning: When using coastlines_planet.osm.pbf, mkgmap needs a *lot* of memory. I found that even for small tiles, 4.5GB of RAM are consumed by the Java process. With coastlines_europe.osm.pbf, the memory consumption is about 2.5GB. - Bartosz [1] http://www.mkgmap.org.uk/pipermail/mkgmap-dev/2010q4/009636.html [2] http://www.mkgmap.org.uk/pipermail/mkgmap-dev/2011q1/010138.html [3] http://www.fabianowski.eu/osm/coastlines/