[Tilesathome] t at h server next generation

[Alan Millar]

> captionless is only z12

We discussed this recently and came to the conclusion that the captionless
layer should exist for more levels than just z12, so that we could do
things like support labels in other languages.  See:

http://lists.openstreetmap.org/pipermail/tilesathome/2008-June/002315.html

> But then I plan to not store empty z12 tilesets at
> all, but have a "blank tileset db" for that (which only has entries for
> completely empty tilesets). But given that we currently waste tons of
> diskspace for the millions and millions of files and thousands of
> directories (which all use up an unknown minimum filesize), I would think
> that we waste still much less space than we currently do.

The blank tile database will be the biggest savings in space of anything
we can do, particulary if it is fully recursive.

There are 16 million tiles at z12, and 13 million of those are sea.  If we
support recursive blankness (such as one z3 blank entry instead of 262144
z12s), all of the blank sea can be stored in a few hundred thousand tile
entries.

Here is a recursive breakdown of the oceantiles data:

Count Zoom Type
     21 3 sea
      4 4 land
     29 4 sea
     44 5 land
    137 5 sea
    157 6 land
    353 6 sea
    510 7 land
    923 7 sea
   1503 8 land
   2225 8 sea
   4547 9 land
   5200 9 sea
  13332 10 land
  12142 10 sea
  34945 11 land
  27812 11 sea
  86587 12 land
  64455 12 sea

Nearly all of the sea will be blank.  Much of the land won't be blank, but
with our Mercator projection, there are a lot of blank land tiles in the
North.

If we can't do full recursion, then hopefully we can at least do z6 to
eliminate 4096 blank z12s.  In addition to the ocean, there is quite a bit
of Yukon, Greenland, and Siberia that can be stored in blank z6 entries.

- Alan