[Mapcss] Zoom levels: bad idea?

[Chris Browet]

On Fri, Jul 23, 2010 at 01:18, Komяpa <me at komzpa.net> wrote:

> 2010/7/20 Chris Browet <cbro at semperpax.com>:
> > Unless I'm mistaken, I understand the zoom level (as in "way|z1-11") as
> "OSM
> > TMS zoom level", right?
> You may understand it that way if you want to.
>

Yo mean the interpretation is up to the implementer? It might be difficult
to have consistent maps for a given style this way...

>
>
> > What about maps rendered in another projection than mercator/google, e.g.
> > OSGB36, which is non-othogonal ?
> They also have a bbox, and may have tiles (at least virtually), thus,
> have zoom levels too.
>

In this case (and in 90% of the non lat/lon-mercator projections) the bbox
is not orthogonal/rectangular (have a look at
http://en.wikipedia.org/wiki/Map_projection for some exotic ones).
The basic assuptions of tiles is that the world representation is
rectangular, which is mostly not the case for projections. Which is why TMS
only accepts lat/lon (EPSG:4326) and mercator as projections...

>
> > How is the implementation supposed to deduce the zoom level based upon a
> > pixels per meters zoom level?
> I guess defining that equator length on z0 is 256 pixels (.26 mm per
> pixel) should be enough. Or, we may leave that up to implementer.
>

Again, we'll loose consistency if we don't define it in the specs/leave it
to the implementer.
And defining a scale in terms of "tiles" would be very, very
abstract/bizarre for people not knowing mapnik/TMS.

>
> > Extreme, but I tend to think the zoom level namespace selector should be
> > dropped altogether in favor of the "way|s5000" indicated as example in
> the
> > specs, which is far more explicit.
> That might be an alternative, but I'd prefer keeping zoom levels
> as-is. If you look at most OSM stylesheets, they first do a conversion
> of sacle into a zoom level and then use zoom levels as constants in
> that case.
>

Zoom levels are fine (as an alternative, IMHO) if they are very clearly
defined in a generic/projection agnostic manner AND if scales are defined
too.
The only generally "recognized" zoom levels are map scales (e.g. 1:5000)
which are clear and precise for everyone. When you buy a printed map, the
scale is not defined by a zoom level ;-)

- Chris -
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