I've been working on making various improvements to the mkgmap
splitter. It was originally intended to split a map into smaller
pieces which could then be converted into Garmin format and artifacts
of that original purpose continue to exist. (Eg, its coordinate system
for representing the areas.)
However, I've committed several improvements to crosby_integration
branch in mkgmap SVN that massively increase its scalability and may
make it more useful to a general audience:
On a 8gb core i7 machine, the new version can:
Split the whole planet into 1200 tiles in about an hour (from pbf
to osm.gz format) in a single pass.
Split the whole planet into 20000 tiles in about 4 hours over 3 passes.
There are some limitations, both in the implementation, and design.
Due to inherent serialization of the internal tracking tables, it
only uses a few CPU cores.
The code uses bounding boxes, not bounding polygons. In the
original purpose of generating mkgmap tiles, tiles are square there
are no polygon tests. Polygon support is straightforward to add, but a
fileformat must be defined to specify bounding polygons instead of
bounding boxes.
At thousands of tiles being output concurrently in a pass, memory
consumption in output buffering can be significant.
The process assumes that the file is sorted with nodes first, then
ways, then relations. Efficiency degrades if they are not further
sorted based on ID.
Ways are not 'completed', A way is included if any node of it is
within the bounding box. Other nodes in the way are not included
unless they too lie inside the bounding box. (So, typically, the
bounding box is expanded to include an overlap region.)
All metadata is dropped.
The code hasn't been generalized to overcome these limitations, but
might be useful to those creating lots of excerpts or breaking maps up
by continent, nation, state, or county. As I don't expect to have much
time to work on the code, I mention it now.
Scott
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