[OSM-talk] Wrong scale in slippy map

[Andy Allan]

On Thu, Apr 23, 2009 at 11:45 AM, Peter Childs <pchilds at bcs.org> wrote:
> 2009/4/23 Jacek Konieczny <jajcus at jajcus.net>:
>> On Thu, Apr 23, 2009 at 08:23:16AM +0000, Jukka Rahkonen wrote:
>>> not mean that these countries are twice as large in the real life.  Scale bar
>>> values, if presented in meters/feet, should be adjusted according to latitude.
>>> Even then it cannot be correct for the whole map, but showing the corrrect scale
>>> for the middle latitude of a would perhaps be the best compromise.
>>
>> If we could get two scales, at the top and at the bottom of the map,
>> then it would give us even more correct information. But one scale will
>> be enough, when correct. Incorrect meter/feet scale is quite useless and
>> probably better would to not use such scale at all.
>>
>
> Call me stupid but are we not going to need a different scale bar for
> North - South as well. Its like putting a Standard 1024x768 display on
> a wide screen monitor everything gets stretched.
>
> You could have more tiles at the equator than at the poles. Dreaming
> that the world can be stretched to fit on a square is always going
> have a problem somewhere.
>
> Ideally you want a map so where a fixed distance is a fixed number of
> pixels on screen and angles are correct at least for the bit of the
> world you are currently looking at.... This should be doable at least
> for higher zoom levels,
>
> How you do this I'm not 100% sure.

If you want it done right, then your scale bar has detatchable end
points. You drag one end of the scale bar to one end of the feature,
the other to the other, and then the scale bar warps itself into a
great-circle curve and tells you how long it is.

Everything else is a compromise :-)

Cheers,
Andy

PS altering the scale bar whilst panning is possible, it's something
we've done in our Web Maps Lite javascript API here at CloudMade. See
http://maps.cloudmade.com for an example. But it's still an
approximation, especially where the distances are significantly
different at different parts of the viewport (it can only give one
scale at a time!)