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

Mon Aug 29 08:43:33 BST 2011

On Mon, Aug 29, 2011 at 10:08:26AM +0300, Bartosz Fabianowski wrote:
>This is why I collected the coastlines for all of Europe in a single 
>file. When all European coastlines are available, you no longer have to 
>tweak tile boundaries.

There are a few drawbacks to that approach. First, you will have to 
download and process much more data than just the country extract.  
Second, if the continent extract was generated at a different time than 
the country extract and there have been edits to the coastline, your map 
might not correspond to any snapshot of the OSM database.

>>I think that it could be useful to have the tile-splitter support a 
>>set of fixed tiles that it would split itself further as needed.
>
>I presume you would then use this so that you extract a larger region 
>and then let the splitter generate tiles for a subset of this region?

Right, the idea is to make the combined bounding box of the 
splitter-generated tiles something else than a rectangle. If we ensure 
that all coastlines that were "cut open" in the Geofabrik extract will 
have their endpoints outside the splitter-generated coastline, the 
flooding should be gone.

>With a sufficient margin of safety between the two regions, this should 
>work by providing the splitter with coastline data even when tiles grow 
>due to rounding.

Right.

I wonder if Geofabrik or some other entity could start providing a 
little wider coastline extracts that are in sync with the Geofabrik 
country extracts. Boundary extracts could be nice too, but I suppose 
that the Geofabrik country extracts already include all the 
boundary=administrative lines of the country.

	Marko