i'm starting to wonder if we're talking about 2 separate issues that lead to similar outcomes.

coastlines break because:

1. they get broken in the extraction process because the poly is too tight
2. they get broken in the splitting process

a separate coastline file would quite likely mend both issues in the same go by creating the coastline out of external data. only drawback: if the poly for extracting the country is too tight, then there'd be an empty area between the coast and the rest of the map data, so all the beach bars are missing.

Am 29.08.2011 09:03, schrieb Bartosz Fabianowski:

geofabrik extracts are to blame - they're too "tight" in some places, so
the coastline breaks in the extraction process already

In a sense, this is true. However, *no* polygon can ever be guaranteed to contain enough data. Even if Geofabrik used a bounding box instead of a bounding polygon so that data for the entire region containing the extract was available, coastlines would still break. It is the rounding of tile boundaries in the splitter that pushes them beyond the region of the original extract. The only robust solution is to extract coastlines separately, for a larger region of the map.

I generated poly files for these countries which simply extend roughly
10 km into the neighbouring countries and the broken coastlines vanish,
even if the splitter splits the coastline in these places.

Do you use these poly files to extract coastlines only or do you use them for all your data? In the former case, you are doing what I found to be a good workaround. In the latter case, you are increasing the region for which coastlines are available while also increasing the region for which tiles are built. If the coastlines happen to work with the new regions, that is great - but it is coincidental, not guaranteed :(.

- Bartosz