[Mapcss] MapCSS 0.2 finalization - zoom levels

Paul Hartmann phaaurlt at googlemail.com
Fri May 4 17:03:14 BST 2012


On 05/01/2012 03:59 PM, Richard Fairhurst wrote:
> 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.
> 
> My long-term intention for the zoom selector was that "z" would not be
> the only way of specifying a zoom level, but that we could also have "s"
> for a standard ratio scale.
> 
> So:
> 
> |z14-18        # "900913" zooms 14-18
> |s25000-50000  # 1:25k to 1:50k

For a map that is displayed on screen, a scale such as 1:50k is not
enough, because you usually don't know the monitor resolution.

We could either use a scale in cm/px directly, or assume some arbitrary
pixel size, like 0.28 mm.

> I think we absolutely need to keep the z selector as it is. Otherwise,
> there is no way to work out what features to include in a simplified
> database for small-scale rendering, bearing in mind that 900913 maps
> represent 90% of the webmaps in use today. But offering an alternative
> for those who want it makes sense to me.

I see that this simplicity is desirable for a TMS using 900913 (aka
EPSG:3857).

But it gets problematic when the map is based on a different projection.
Are you supposed to replicate the 900913 distortions? This is hard to
swallow.

Paul



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