[mkgmap-dev] Coastline issues - analysis and possible solution

Mon Aug 29 08:03:48 BST 2011

> 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