[OSM-dev] Complete tileset - experiments with tile sizes and disk space

OJ W ojwlists at googlemail.com
Mon Dec 22 21:00:49 GMT 2008


On Mon, Dec 22, 2008 at 1:43 PM, Bill Magee <bill at billmagee.co.uk> wrote:
> I don't do *nix but on WinXP the typical allocation size for large disks
> is 4kb (and I assume other file systems will do similar). Any tile under
> 4kb (lots of them) will occupy this 4kb regardless.

Back when tiles at home stored its images in one file each, I think the
sysadmins chose a block size of 512 bytes for exactly that reason.
That would have been on Linux with ReiserFS though, so probably
doesn't help for your app.

The replacement scheme was
http://wiki.openstreetmap.org/wiki/Tiles%40home/Tileset_as_one_file
which puts 1366 tiles in each file (along with compression for various
types of blank tiles) - apparently it doesn't take long to extract the
PNG data for any given tile from such files.

Depending on what you're rendering, you may find that most tiles are
blank (more so for specialist layers like bus routes, less so for
layers with autogenerated content on like contours) so it's worth
considering those when calculating the expected disk usage.
(plain-colour 'blank' tiles might be sea or forest etc., as well as
just empty land, depending on your rendering)




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