Each year English Heritage undertakes a programme of aerial reconnaissance covering the whole of England. The reconnaissance is undertaken by two Aerial Survey teams based in the Swindon and York offices. The York office staff cover the northern counties (Cumbria, Northumberland, Durham, Tyne and Wear, Cleveland, North Yorkshire, East Riding of Yorkshire, West Yorkshire, South Yorkshire, Lancashire, Merseyside, Cheshire, Staffordshire, Derbyshire, Nottinghamshire and Lincolnshire).