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[mkgmap-dev] Sea tiles problem

From Christian Gawron christian.gawron at gmx.de on Tue Oct 27 20:31:04 GMT 2009

Toby Speight schrieb:
> 0> In article <4AE592A2.7060209 at gmx.de>,
> 0> Christian Gawron <URL:mailto:christian.gawron at googlemail.com> ("CG") wrote:
> CG> Could you provide me a download link for the offending tile?
> I don't have a server I can put it on, but I'm using Geofabrik's "Great
> Britain" extract[1], and this is the relevant line from the splitter:
> /--------
> | 63240015: 2615296,-407552 to 2846720,102400
> | #       : 56.118164,-8.745117 to 61.083984,2.197266
> \--------
> [1] <URL: http://download.geofabrik.de/osm/europe/great_britain.osm.bz2 >
Ok, I will have a look at it. I'm not so familiar with the newest 
version of the splitter - how can I tell it to create the tile 63240015 
with the coordinates above? Can you send me the areas.list?
> CG>
> CG> The algorithm I implemented handles two cases of land masses:
> CG> - closed shoreline segments (aka islands)
> ... or inland seas (if clockwise)?
Are there really inland seas using natural=shoreline instead of 
> CG> - shoreline segments which are not closed but extend to the border
> CG>   of the tile (i.e. the intersection of a closed shoreline with the
> CG>   bounding box)
> CG>
> CG> In the latter case the shoreline is "closed" along the tile border.
> CG> The algorithm then generates a sea background and a multipolygon
> CG> relation which adds a "hole" for each landmass.  Obviously a landmass
> CG> with a non-closed boundary will be drowned by that approach :-)
> I would expect any non-closed coastlines to show up in OSM Inspector,
> and I'm not seeing anything there.  
No, they are usually a result of some cutting process.
> I'm wondering how the closure of cut
> shorelines is done (having not yet looked at the code), and perhaps it
> is turning right instead of left for some reason?
No, the turning direction should be correct.
> Also, in passing, is it normal to have lots of little blue "threads"
> appear on the GPS display connecting the sea regions?  I've not seen
> this with other multipolygons, so I'm wondering if this is also a
> symptom of problems.
No, this is part of the multipolygon code. It connects the inner and 
outer polygons with two lines, and the coordinates of the "returning" 
line are move by one map unit. There is a comment in the code stating 
that this results in "triangles" but that it is necessary or some 
"inner" polygons are not rendered at all. I must admit that I don't 
really understand how polygons with "holes" should be coded in the IMG file.

Best wishes

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