[OSM-talk] tiles at home 404 handler, and assumed seas

[Frederik Ramm]

Hi,

    the old lowzoom.pl depended on the ocean tile index file to know 
wheter it should use blue water when the server didn't return anything. 
Since the server now always returns something, I removed that code, and 
rendered four level-7 tiles aroun the Orkney islands.

Looks funny:

http://informationfreeway.org/?lat=8200145.76009&lon=-358413.14795&zoom=7&layers=0B00000

Especially with most of Scotland under water ;-)

I think the "404 assumed sea" text should go (or become very very small 
and "dark shade of blue on light shade of blue") so not to disturb the 
level-7 tiles. Reinstating dependency on the tile index (i.e. not 
believing what the server tells us) would not be too nice because then 
level-7 and level-12 would have potentially different ideas of where 
water is, leading to all sorts of confusion when zooming in.

Scotland being under water in this example means that the following idea:

 > If I provide you with a list of all level-12 tiles on the earth's
 > surface that are "land" but not covered by a real tile, would you use
 > that to seed your database?

really should be done. It may be true that most of the world is water, 
but still there are about 3.2 million land level-12 tiles and we only 
have data for less than 100k of them, so defaulting to water makes us 
wrong for 3.1 million level-12 tiles ;-) Of course we were even more 
wrong before when we had no water at all, but it was not as obvious!

We can either put the (roughly) 3.2 million land tiles in the database, 
or we can continue the recursive process sketched here

>      if(mysql_num_rows($Result) == 0){
>        if($Z <= 12)
>          BlankTile("assumed_sea");// political decision: make unknown 
> areas blue
>        else
>          SearchDatabase(int($X/2),int($Y/2),$Z-1,$LayerID);
>      }

to smaller zoom levels (so that we only have to store one database row 
if the whole of a level-7 are is land, for example). That would require 
only about 114k entries, not 3m, but be less efficient upon retrieval.

If you tell me which one you want, I can prepare a file with insert 
statements.

Bye
Frederik

-- 
Frederik Ramm  ##  eMail frederik at remote.org  ##  N49°00.09' E008°23.33'