Created in the time of Geoffrey Chaucer, the 14th-century Gough map of Britain was a landmark in map-making and was still in use 200 years after it was drawn. But what do the red lines signify along the coast, and why is there a massive green wilderness dominating the centre of Wales? While exploring these mysteries, Nicholas Crane attempts to use the map to find his way on foot from Snowdon to St David's.