[OSM-talk] Mapnik - Town names at zoom level 12

[Jon Burgess]

On Tue, 2007-06-19 at 12:01 +0100, Abigail Brady wrote:
> On 6/19/07, Andy Robinson <Andy_J_Robinson at blueyonder.co.uk> wrote:
>         Is there a logical reason why the town names of Tamworth and
>         Nuneaton would
>         not appear on Mapnik level 12? The names appear on other zooms
>         either side
>         and town= gets rendered at zoom 12 for plenty of other towns
>         that I can see. 
>         Both have been in the db for quite some time.
> 
> Not sure exactly what's causing it, but this is also evident in
> central London, where for some reason it's not showing "City",
> "Camden" or "Hackney" at z12 even though there is plenty of space.
> Similar, but different, missing labels happen on z13. 
> 
> As an aside, I've added a few population= tags to large towns and
> small cities in England and Wales, so if the city-picking logic in
> renderers could be changed from matching place=city to 
> 
>    (place=city && (undefined(population) || population >= 50000)) 
> || (place=town && population > 100000))
> 
> this would result in somewhat better results at lowzooms (ie it'd get
> rid of Chichester, Lichfield and Ripon and add Guildford, Telford and
> Middlesbrough...). 

I think what is happening in both cases is that the text is colliding
with a red dot of a train station. It looks like the red dot wins and
prevents the name from being rendered. If I turn off the rendering of
amenity=station then extra text labels appear. I guess the high density
of underground stations in central London explains why the problem
occurs so frequently.

I suppose the text placement in Mapnik could be changed to allow more
movement of town/city labels but I don't know the internals of how
algorithm operates.

	Jon