[Mapcss] MapCSS 0.2 finalization - zoom levels

[Paul Hartmann]

On 05/04/2012 03:00 PM, Komяpa wrote:
> 2012/5/1 Richard Fairhurst <richard at systemed.net>:
>> On 01/05/2012 14:47, Paul Hartmann wrote:
>>>
>>> In my opinion, it should be the same in MapCSS and the definition of the
>>> zoom selector should be based on map scale rather than on the projected
>>> coordinates in the Mercator projection.
> 
> Here we come to usual problem:
>  - those who came from traditional maps think in scales;
>  - those who came from web-GIS think in zoom levels.
> 
> There's no useful "scale" in EPSG:3857 - it changes depending on
> latitude. So it is possible that renderer has to mix styles for
> different scales in one screen.

This is maybe too drastic, I'd just take the scale of the centre pixel
and use it for the entire mapcss styling process.

> If you're using a different projection, and it's majorly different
> from EPSG:3857, you have no "default" tiles grid.
> 
> Do we have any real application now that uses not EPSG:3857 aka
> Spherical Mercator for rendering?

In JOSM you can switch to different projections, and this is often
necessary in order to display aerial imagery from a local WMS.
(On-the-fly warping isn't supported for WMS.)

Spherical Mercator is just one projection of many and if it was the
reference point, I'd had to put

mapcss_scale = scale * cos(lat)

somewhere in the display code, and this feels a bit arbitrary.

On the other hand it is good, when the mapcss zoom levels correspond to
the TMS levels, if they are displayed in the background.

Paul