[OSM-dev] Sea tiles to database

David Groom reviews at pacific-rim.net
Wed May 2 22:35:47 BST 2007


----- Original Message ----- 
From: "OJW" <streetmap at blibbleblobble.co.uk>
To: <dev at openstreetmap.org>
Sent: Wednesday, May 02, 2007 7:12 PM
Subject: [OSM-dev] Sea tiles to database


>
> Unless there are any other suggestions, I propose:
>
> 1) database:
> * creating a database of "all-sea" tiles
> * populating it with the list of z-12 tiles
> * deleting the images
>
> That will save a few multiples of 1024 bytes (maybe 13,000,000 of?)
>
> 2) database access in 404 handler
> * search on disk first for real tile
> * if none found, use database to search for a sea tile
> * if no sea-tile found, return white
>

Would it not be better to assume something is sea unless other information 
says not. Then database would store "land tiles" not "sea tiles".

There would I'd have thought been two reasons for this:

a) the world is 71% sea so that should be the default;
b) in time in theory most of the land will be represented by "real tiles" 
with actual osm data in them, these tiles could be removed from the "land 
tile database" and the amount of "land tiles" data stored in the database 
falls even lower than the 29 % of the world which is not sea.


> 3) optimise sea database
> if someone has a workable idea for binary files to store the sea in, it 
> can be
> exported regularly from the database into that format, to allow faster
> lookups while retaining the versatility of a database.
>
> if not, we can still do things like
> * merge 4 adjacent sea tiles into one at a 'lower' zoom level
> * delete sea tiles whose parent is also sea (see hierachical database 
> message)
>
> As a first pass, this will only affect existing tiles.  The next step will 
> be
> for upload handler to create sea tiles directly instead of storing them as
> images. (discussion for a different email thread maybe)
>
> .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 
> .  .  .  .  .  .
>
> To locate the z-12 sea tiles, I've extracted the data from today's
> oceantiles_12.dat file on svn[1].  I've generated an image[2] showing the
> results. These are the files that will get deleted, so shout if you can 
> see
> any errors in the data or methods used to extract it (source code 
> online[3])
>

before going live on the tiles at home layer  would it be possible to add 
another layer to www.infomationfreeway.org showing what is going to be 
rendered as sea, so that we could check this against the known coast?

David



> [1]
> http://svn.openstreetmap.org/applications/rendering/tilesAtHome/oceantiles_12.dat
> [2] http://dev.openstreetmap.org/~ojw/temp/seatiles.png
> [3] http://dev.openstreetmap.org/~ojw/temp/
>
>
> Regards,
>
> OJW






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