[OSM-dev] map projection and scale distortion

Raghu Ram raghuram.nadiminti at gmail.com
Wed Apr 16 18:09:10 BST 2008


sorry for asking it again but .......

Also I see that Mapnik osm.xml (style sheet) filters on the basis of scale
denominators. But assuming that the projection suffers some scale distortion
how could one ensure that the same style is applied all over the world at a
particular zoom level. I am trying to develop a WMS SLD ( projection is
EPSG:4326 ) and facing issues with scale denominators and zoom levels.
Please help me out how OSM handles this issue.
Thank You

On Wed, Apr 16, 2008 at 10:27 PM, David Earl <david at frankieandshadow.com>
wrote:

> On 16/04/2008 17:44, Christopher Schmidt wrote:
> > On Wed, Apr 16, 2008 at 05:26:16PM +0100, Tom Hughes wrote:
> >>>  I flushed my browser cache. Still i cannot see any dynamic scale bar.
> Its
> >>> static in the sense that the scale doesn't change with the latitude in
> >>> focus.
> >> Well I guess that's a bug in OpenLayers then ;-)
> >
> > "Bug" is so harsh. "Lack of Feature" is so much nicer. :)
> >
> > Seriously though, it's a difference in how you look at the problem. All
> > of the code in projected maps anywhere assumes that within a plane,
> > units are planar. This usually means that measuring large scale
> > distances is not within the design. This is the case for the Scale
> > control as well.
> >
> > Implementing this as an option would not be difficult (Though it would
> > require some discussion): Anyone interested is encouraged to talk to the
> > OpenLayers Mailing List or IRC channel about it.
>
> Seems to me like it is useful at and above some zoom level where
> horizontal and vertical are deemed near enough equal in the Mercator
> projection for the error to be insignificant.
>
> BTW, I think the little bar is too short to be useful. It could do with
> extending some way across the bottom of the map, with divisions at the
> size it shows now. Ideally it wouldn't stray above the 'Permalink' link
> so that it doesn't reduce the area one can screenshot usefully at one
> time without edge decorations.
>
> David
>
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