Exmoor is loosely defined as an area of hilly open moorland in west Somerset and north Devon in South West England. It is named after the River Exe, the source of which is situated in the centre of the area, two miles north-west of Simonsbath. Exmoor is more precisely defined as the area of the former ancient royal hunting forest, also called Exmoor, which was officially surveyed 1815–1818 as 18,810 acres (7,610 ha) in extent. The moor has given its name to a National Park, which includes the Brendon Hills, the East Lyn Valley, the Vale of Porlock and 55 km (34 mi) of the Bristol Channel coast. The total area of the Exmoor National Park is 692.8 km2 (267.5 sq mi), of which 71% is in Somerset and 29% in Devon.