[Marble-devel] Coordinate System of GeoPainter, GeoDataCoordinates and similar

Torsten Rahn tackat at t-online.de
Tue May 15 02:18:56 UTC 2012

Hi Arne,

On Montag, 14. Mai 2012 19:57:31 Arne Krüger wrote:
> Hi,
> I am fairly new to Marble and wondering which coordinate system is used
> by classes such as GeoPainter and GeoDataCoordinates and how this
> relates to different map projections. I probably will mostly use
> OpenStreetMap which - as far as I know - uses a Mercator-On-Sphere
> projection just like Google and Bing. However, since my application
> requires drawing WG84 data onto the map I wonder whether I need to
> transform my GPS coordinates before passing them to the Marble classes.

Marble just assumes coordinate data with WGS 84 datum to be used. This is for 
consistency with other data (such as OpenStreetMap) which is also provided 
with WGS 84. 

Apart from that there is no handling for other geodetic reference systems 
inside Marble. 

To visualize the data we provide support to display it on a Sphere ("Globe"), 
on a Plate Carre projection (also named "Equirectangular" in the Marble Source 
Code and referenced as a "Flat Map" in the UI) and for Mercator. 

This kind of projection handling is done automatically by Marble's classes 
such as GeoPainter or the texture mapping classes that allow to map tiled 
bitmap data onto the globe.

So if you create placemarks or if you paint using GeoPainter you don't have to 
worry about projections. 

For tiled bitmap data there are basically two source projections supported:

"Mercator" (as used by OpenStreetMap and Google Maps) and 
"Equirectangular/Plate Carree" (as used by other maps such as Blue Marble).

There is a short tutorial here that provides more information on how to 
integrate tiled bitmap data:


> PS: I would highly appreciate a link to the API documentation on the
> main project page, since the latter has a rather prominent google search
> ranking, which the former is sadly lacking.

Yes, we should at least reference it on the Marble Techbase Page.

Best Regards,



> Thanks for hints and clarifications.
> Arne
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