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

Mon Aug 29 15:44:30 BST 2011

> i'm starting to wonder if we're talking about 2 separate issues

Indeed, it seems so :).

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

You mean the bounding polygon is so tight that it actually clips away 
the coastlines? If so, the bounding polygon is simply wrong. It is no 
longer *bounding* but cutting into the country. This would definitely 
damage coastlines. And yes, you are right - this is not the problem I 
was referring to.

> 2. they get broken in the splitting process

This is what I meant. More specifically, the coastlines stay as they are 
but the map area increases so that there are suddenly regions in the map 
for which no coastline data is available, breaking the subsequent sea 
generation.

> 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.

Issue 1 described by you is one of missing data. A separate coastline 
file would be able to restore coastline data but would be unable to fill 
in the gaps, just as you say. I think that the only way to address issue 
1 is by fixing the bounding polygon. If issue 1 happens with Geofabrik 
extracts, I am rather sure Fred would like to hear so he can fix the 
polygons used.

For issue 2, which I found to be quite common throughout Europe, 
separate coastlines are a correct solution. This is what I suggested 
they be used for.

- Bartosz