[OSM-dev] Two technology related questions

[Tom Hughes]

On 30/01/14 10:47, Peter Wendorff wrote:

> Dirty tiles are rerendered depending on their zoomlevel. The
> low-zoom-levels (z0..5 afair) are rendered usually independent of tile
> requests as they use a lot of data for the same image size and therefore
> rendering takes too much time (and of course they would be dirty very
> often as they span across a big area of the world).

Actually it's all the way up to z12 that is normally prerendered and not 
updated on demand. That works by just not marking any tiles between z0 
and z12 as dirty when changes happen.

> As far as I know all related/covereage tiles are marked as dirty, but
> the low zoom levels are not rerendered on request. So if I am up to date
> there is no decision based on attributes (e.g. zoom level 2 is not
> affected because there's only a highway-tag which changed and highways
> aren't shown on z2).

Actually we use a quick-and-dirty method of identifying which tiles have 
changed so I believe that relation changes may not necessarily mark all 
effected tiles as dirty.

There is a system where osm2pgsql identifies the changes as it loads the 
update which is more accurate but we don't use it, I think because it 
tends to mark so many tiles as dirty which are only marginally affected.

Tom

-- 
Tom Hughes (tom at compton.nu)
http://compton.nu/