[OpenLayers-Users] measure control: google projection metric or not ?

[Andreas Hocevar]

On May 11, 2010, at 12:29 , Oliver Christen wrote:

> Hello
> 
> I had to do an intervention on a project using google projection 
> (epsg:900913).
> That projection is supposed being metric, but to be able to get correct 
> measurement values (in meters) with the OL Measure control, I had to specify 
> "geodesic" as true in the controler's config.
> 
> why ?
> is the google projection metric or not ?

The projection uses meters as distance units, yes. But like with every projection, measuring lengths is only accurate at some very special places. For Spherical Mercator, this is along the equator. Mercator projections are optimized for measuring angles, which is important for nautical purposes. Other projections are optimized for measuring areas, which is important for cadastral maps.

Anyway, the more you move away from the equator in Spherical Mercator, the less accurate your distance measurements get. And "geodesic measurement" is nothing else but measuring on the spheroid instead of the projected map.

Regards,
Andreas.

> 
> thanks in advance
> regards
> Oliver 
> 
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-- 
Andreas Hocevar
OpenGeo - http://opengeo.org/
Expert service straight from the developers.