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[mkgmap-dev] Sea tiles problem

From Toby Speight T.M.Speight.90 at cantab.net on Tue Oct 27 12:03:50 GMT 2009

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 >

CG> Did mkgmap print a warning message "Non-closed coastline segment does
CG> not hit bounding box: ..."?

I didn't see any, but they may have been lost amongst a slew of "zero
length arc" messages from the roads.  I'm re-running the build, so I can
throw grep at the logs.

[time passes]

No, definitely no occurrence of the word "coastline" in the output.

CG> The algorithm I implemented handles two cases of land masses:
CG> - closed shoreline segments (aka islands)

... or inland seas (if clockwise)?

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> 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.  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?

CG> One limitation I know of arises with country extracts from geofabrik -
CG> they have shorelines cut-off at the country border.  As UK is a big
CG> island, this should not be the cause of your problem, though.

One big island and several hundred small islands, but yes, there's no
coastline hitting the edge of the extract (Ireland is not included).

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.

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