[OSM-dev] map projection and scale distortion

Christopher Schmidt crschmidt at metacarta.com
Wed Apr 16 17:56:49 BST 2008


On Wed, Apr 16, 2008 at 06:42:33PM +0200, Iván Sánchez Ortega wrote:
> 
> <quote who="Tom Hughes">
> > Iván Sánchez Ortega <ivansanchez at escomposlinux.org> wrote:
> >
> >> Yes, the slippy map scale should change with the latitude in
> >> focus.
> >
> > Well that rather depends really - the amount the scale changes by
> > depends what direction you are measuring in.
> 
> And even more, it depends on the pixel you're looking at - pixels at the
> top of the map have a different scale than pixels at the bottom of the
> map.
> 
> I dare to raise the question: Does a scale bar make any sense at all in a
> map with an spherical mercator projection?

It's somewhat helpful when you're working at a 'local' level: when
you're in the downtown of London, or Boston, it's good to offer an idea
of how big something actually is. The value is similar to the value
added by the large 'panzoombar': it lets you know what 'scale' you're at
to a certain extent, for users who don't intuitively know how bug
something at zoom 17 is, for example. 

Does it make much sense at a worldwide level? Not really. But people
seem to love to drag the map arond and see the size of the scalebar
change.

Regards,
-- 
Christopher Schmidt
MetaCarta




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