The first map produced by the Ordnance Survey, their blog reminds us, was this map of Kent. Published in 1801 at the scale of two inches to one mile (1:31,680), it took three years to complete; the OS started in Kent over fears of a French invasion. As such, the map “focused on communication routes and included hill shading to ensure men at arms could interpret the landscape with precision. Over time, this map design became less focused on these elements and was developed to appeal to a much wider audience.”
The definitive history of the early years of the Ordnance Survey is Rachel Hewitt’s Map of a Nation, which I reviewed in 2012.