[mkgmap-dev] Splitter question

Mon Nov 22 17:02:32 GMT 2010

On Sun, Nov 21, 2010 at 1:04 PM, Danny Backx <danny.backx at scarlet.be> wrote:

> I've taken some time and played with the suggestions given.
>
> I'm including two scripts. One (doit.nl) is a sample script to split the
> OSM file for the Netherlands into manageable pieces, and then to convert
> the OSM files into RoadMap format. This mostly works :
> - I get 339 map files
> - 9 maps turn out to be too big still for RoadMap
>
> So I created another script (deeper.nl, used in a subdirectory, you'll
> notice that in the script) that'll take the files I put in the
> subdirectory, and divide them in four smaller chunks.
>
> I used deeper.nl and still had problems with 3 of the smaller chunks so
> I reran deeper.nl one more time, producing maps that were (again) four
> times smaller than the offending OSM files.
>
>
>
> I realize that I'm working around something splitter is leaving
> unsolved, and I'm partially automating this, my question is whether all
> this is a good idea.
>

If you're willing to do some work on the splitter to improve it, I think
that yes, you could improve it in a few days of work to the point where this
becomes a non-issue for you, and the splitter generates excellent and
well-balanced tiles.

The core problem is that the splitter needs to track the point density
around the world very densely. It, divide the world into, say 8192 tiles
horizontally and 8192 vertically. It goes through the planet it counts how
many nodes there are for each tile, and then uses that summary 'density map'
table to derive the areas for splitting, by, say. Your problem is that you
want regions smaller than 1*1, because even a 1*1 has too much data for your
program. Increasing the density that stuff is tracked, to 32k*32ktiles
across the world increases memory by 16x and might not solve your problem
because the the minimum sized tile of 1*1 may still have too many nodes,
even if it is smaller in geographic size.

The fix for you is that dividing Europe into, say 16k*16k tiles generates
tiles with a smaller geographic extent than dividing the world into tiles of
32k*32k. You should be able to feed the geographic bounds data from the
<bound> tag to configure the density map to only cover part of the world.
There are a few catches in that you'll have to obey the alignment
constraints that are required by mkgmap. (Which, alas, I do not know.). But
that would solve your problem and might make the splitter more useful to
others..

There are also a few smaller-scale things that may help. For instance, the
minimum are size in the splitter is 2*2 tiles, not 1*1 tiles.

Scott
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