Each Past & Present map is centred on a city or town and covers an area of 15km x 15km using four Ordnance Survey mapping series to cover four different periods: the mid 19th century, the late 19th century, the1920s and the present-day. All four maps cover exactly the same area. To achieve this, the three historical maps have been enlarged from their original inch-to-the-mile to match the 1:50,000 scale of the present-day one.