[OSM-talk] scale question

Ben Last ben.last at nearmap.com
Fri Jul 9 01:27:29 BST 2010


>
> At 09.07.2010 01:19, Mostafa El-ashram wrote:
>
> Please could you tell me how to determine the scale of a map on open
>> street maps
>>
>
The TMS co-ordinate system (OSM,  Google Maps, NearMap, Bing Maps, etc) use
a Mercator projection; you'll find it referred to as "cylindrical Mercator"
and "spherical Mercator".
http://en.wikipedia.org/wiki/Mercator_projection
http://mathworld.wolfram.com/MercatorProjection.html
http://www.sharpgis.net/post/2007/07/27/The-Microsoft-Live-Maps-and-Google-Maps-projection.aspx

To derive the scale in metres per pixel at a given latitude and zoom level,
you can use this (JavaScript, which uses radians for trig):

metresPerPixel = Math.abs(156543.04 * Math.cos(latitudeAtCentreOfMap *
nml.PI / 180)) / Math.pow(2, zoomLevel);

See http://msdn.microsoft.com/en-us/library/aa940990.aspx for a reference.

Cheers
b

-- 
Ben Last
Development Manager (HyperWeb)
NearMap Pty Ltd
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