Its becase OSG's default shadowMap code isn't very good for large scenes - the 
shadow camera frustum is sized to fit the entire scene, so an individual shadow 
map pixel may cover a large number of rendered fragments, making the shadows 
very pixellated, and the 'shadow acne' - caused by a similar aliasing issue is 
severe.

the 'Soft shadow map' technique does some filtering which improves things a 
fair bit, and there are other algos like LISPSM which try to use shadowmap 
pixels more efficiently.

The 'standard' approach to this in other engines is to use cascaded shadow 
maps, which render multiple passes and combines them to make close-up and 
distance shadows work nicely, but this approach is not implemented in osgShadow 
AFAIK.

heres some info on shadow mapping details:

http://www.opengl-tutorial.org/intermediate-tutorials/tutorial-16-shadow-mapping/

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