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[mkgmap-dev] Sea tiles problem

From Toby Speight T.M.Speight.90 at cantab.net on Wed Oct 28 08:54:09 GMT 2009

0> In article <4AE75888.8060206 at gmx.de>,
0> Christian Gawron <URL:mailto:christian.gawron at gmx.de> ("CG") wrote:

CG> Toby Speight schrieb:

>> 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> Ok, I will have a look at it. I'm not so familiar with the
CG> newest version of the splitter - how can I tell it to create
CG> the tile 63240015 with the coordinates above?  Can you send me
CG> the areas.list?

I could give you the full areas.list (I don't know whether you can make
splitter create a single tile) - but - I'm experimenting with cutting out
smaller extracts using osmosis, to see whether I can produce a smaller
file that still triggers the problem.  I did manage to reproduce it by
extracting just the coastline from great_britain.osm:

| osmosis --read-xml $< --way-key-value keyValueList='natural.coastline' --used-node --write-xml $@

This gives a file containing only the coastline, which can be processed
without splitting.  And, for me, this exhibits the inversion.  It does
take quite a while to run through mkgmap, unfortunately (an hour of CPU
time for me), so I'm looking at cutting it down to process smaller
groups of islands.  I might not get it done today, though, and I'm
about to go off-line for a few days (back around the middle of next

BTW, I was wrong about this affecting multiple areas of the GB extract -
the other boundaries I saw were from my contour tiles, not general OSM.

CG> Are there really inland seas using natural=shoreline instead of
CG> natural=water?

I've no idea - not in my region, I would think, but perhaps the Dead
Sea?  Let's not worry about this for now!

>> 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.
CG> No, this is part of the multipolygon code.  It connects the inner and
CG> outer polygons with two lines, and the coordinates of the "returning"
CG> line are move by one map unit.  There is a comment in the code stating
CG> that this results in "triangles" but that it is necessary or some
CG> "inner" polygons are not rendered at all.  I must admit that I don't
CG> really understand how polygons with "holes" should be coded in the IMG
CG> file.

Okay, that's reassuring to know.  It's a separate issue that can be
dealt with independently.

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