Outward urban expansion is now prevented by the Metropolitan Green Belt, although the built-up area extends beyond the boundary in places, resulting in a separately defined Greater London Urban Area. Beyond this is the vast London commuter belt. Greater London is split for some purposes into Inner London and Outer London. The city is split by the River Thames into North and South, with an informal central London area in its interior. The coordinates of the nominal centre of London, traditionally considered to be the original Eleanor Cross at Charing Cross near the junction of Trafalgar Square and Whitehall, are approximately 51°30′26″N 00°07′39″W﻿ / ﻿51.50722°N 0.12750°W﻿ / 51.50722; -0.12750.