[mkgmap-dev] problem with --generate-sea near Toronto, Canada

Mon Jan 31 22:32:30 GMT 2011

On Sun, Jan 30, 2011 at 9:42 PM, Marko Mäkelä <marko.makela at iki.fi> wrote:
> On Sun, Jan 30, 2011 at 08:53:09PM +0100, Ben Konrath wrote:
>>I've run into a strange problem with --generate-sea while generating a
>>map of Canada. Here's a screenshot of the problem I'm seeing:
>>
>>http://bagu.org/scratch/generate-sea-problem-lake-ontario.bmp
>
> You seem to have have a vertical band of inverted land/sea west from
> Oakville. I would guess that the NW or NE corner of the inverted box is
> at a tile border. I would suspect some error at the spot where one of
> the vertical lines intersects with the real coastline.
>
> Hmm, I was almost going to believe that the ocean starts right south of
> Canada, but then I remembered that there is some piece of land called
> the United States in between. :-)

You can't really see the U.S. across the lake from Toronto it's easy
to pretend it's not there :-).

> I would guess two possible causes for your problem. Either your tile
> boundaries are chosen badly, so that some multipolygons are severed by
> them, or the input data is bad. splitter.jar will leave some 'safety
> margin' around the tiles, but it may not be enough. When the Lake
> Päijänne in Finland was defined in a single multipolygon (it used to be
> natural=coastline), I got some errors that I fixed by moving my tile
> boundaries so that the entire lake fits in a single tile.

The actual coastline seem fine around where the problem flooded land
hits the lake. I think it probably has something to do with the
multipolygon that makes up the North shore of Lake Ontario. I'm using
my own Canada polygon cut from the planet so I might be cutting off
the parts of the multipolygon. I'll look into this when I have some
time.

> If you want generate-sea to work on a non-rectangular map extract (and
> if the Great Lakes have been defined as natural=coastline), you may have
> to apply some black magic when choosing the tile borders.  For my
> Finland map, I extracted the natural=coastline from the Geofabrik
> finland.osm.pbf with Osmosis, and loaded the result in JOSM. Then I
> figured out how to choose the tile borders so that the missing sections
> of natural=coastline would be outside the tile borders. Only in the
> north, I had to use extend-sea-sectors to augment a missing bit, because
> I did not want to create lots of tiny tiles near the Swedish/Finnish
> land border.

Thanks, this is really useful information.

>>I'm using mkgmap version r1783 with these options:
>>
>>java -Xmx400M -Dlog.config=/home/ben/osm/mkgmap/resources/logging.properties
>
> 400M could be a bit scarce, but probably not causing this. I run with
> -Xmx1024m. If your logging.properties is like the one I have at
> http://www.polkupyoraily.net/osm/, you should have some mkgmap.log.*
> files. The most recent one should be mkgmap.log.0.
>
>>Does anybody know what's going on? What's the best way to debug a
>>problem like this?
>
> Search mkgmap.log.0 for 'MultiPolygon' or 'coastline'. Or, if you are
> ambitious, take and adapt my logging.ignore, and grep -vf logging.ignore
> mkgmap.log.0, and fix all errors. That is doable for a small country,
> such as Finland. I update my public map only when there are no serious
> errors (such as multipolygon issues or dead-end oneways other than some
> driveway).

Again, great information. Thanks! I'll check how Canada is doing on
the error front. For now I'm relying on people who download my map to
let me know if something's broken but it's would be better to add some
error checking like you do.

Cheers, Ben