[mkgmap-dev] Land polygon from sea generator

Mon Jun 6 18:03:17 BST 2011

Hi Adrian,

> I have recently updated my setup to use a new version of mkgmap with
> improvements to sea generation and indexing. I would like to
> congratulate all involved for the great progress that has been made.
>
> I made a map of Great Britain and France. It occupies 1.1GB and
> comprises 184 tiles. I extracted the area I wanted from the Geofabrik
> Europe extract. The mkgmap run took 52 min (Core 2 Duo 2.66GHz,
> --max-jobs). I gave it 7GB of RAM and it used 4GB. [I am using Apple
> Java 1.6. Previous releases simply stopped with an error if they ran out
> of memory. The latest release appears to do something similar to virtual
> memory, so it does not stop when it runs out of memory, but it becomes
> extremely slow when it starts to swap to disk.]
>
> I used generate-sea:multipolygon,extend-sea-sectors with mkgmap-r1955
> (trunk) and the sea all seems to have been generated correctly. There
> was just one place where a land polygon was missing, and so I got a
> yellow background instead of white. Tile details from areas.list:
> 63240143: 2381824,49152 to 2439168,131072
> #       : 51.108398,1.054688 to 52.338867,2.812500
> This tile contained some white land and some yellow land. The yellow
> area was around 51.9N 1.1E. It is the area south of the Stour estuary
> including Clacton-on-Sea and Walton-on-the-Naze.

Can you please open the area with openstreetmap in your browser and send 
the permanent link? I want to avoid to have a look at the wrong area. 
(Debugging of the generate-sea parameter is quite laborious)

>
> I inspected the coastline data of this tile in JOSM and I could not see
> any problem with it. This is, however, a very complex coastline. The
> coastline ways and nodes account for about one fifth of the data of the
> tile. I think this coastline contains an excessive amount of detail.
> There appears to have been a large-scale import from the Ordnance Survey
> Vector Map and I suspect the data was not simplified before it was added
> to OSM.
>
> I have uploaded Ipswich.zip to files.mkgmap.org.uk/download. I would be
> grateful if someone could look at why the land polygon was not
> generated. The .zip contains: 143coast.osm.gz, which has just the
> coastline elements from the aforementioned tile, my commands, style and
> .TYP files, the details from areas.list, and the actual mkgmap command.
>
> I tried to use osmosis to extract the coastline elements from the tile,
> but osmosis could not touch the file produced by splitter, not even an
> old version of osmosis which supports API 0.5. Osmosis complained that
> the time stamps were missing. This is what I did instead:
> 1. Load the file produced by splitter, into JOSM. JOSM produces tens of
> thousands of warnings but it opens the file. (I believe JOSM was
> modified specifically to allow it to open splitter tiles.)
> 2. Use the search feature to select all elements tagged
> natural=coastline.
> 3. Create a new data layer.
> 4. Merge the selection into the new layer.
> 5. Save the new layer as 143coast.osm. This file lacks the bounds data,
> so...
> 6. Expand the file produced by splitter, to 63240143.osm.
> 7. Use a text editor to copy and paste the bounds element from
> 63240143.osm to 143coast.osm. (I was surprised that my text editor could
> open a 100MB file, but it did. It took 0.5GB of RAM.)
>
> During the run of mkgmap, three tiles produced errors like this:
> SEVERE (MapSplitter): 63240175.osm.gz: Area too small to split at
> http://www.openstreetmap.org/?mlat=52.03131&mlon=0.87899&zoom=17 (reduce
> the density of points, length of lines, etc.)

This is a known problem of mkgmap. There is much work to do to fix that. 
You might read 
http://www.mkgmap.org.uk/pipermail/mkgmap-dev/2011q1/009960.html and 
http://www.mkgmap.org.uk/pipermail/mkgmap-dev/2011q2/011379.html for 
more info.

> With two of the tiles, I could not see any resulting defects in the map.
> For the third tile -
> 63240175: 2381824,32768 to 2439168,49152
> #       : 51.108398,0.703125 to 52.338867,1.054688
> the only defect I could see was that at resolution 24, the land polygon
> was missing in the part of the tile to the north of 52.03131N, the
> latitude given in the error message.
>
> This run of mkgmap also prompted other questions which I might raise as
> separate threads.
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