This is known as the Red Lined Map.The line which gives it its name can be seen clearly. What does it signify?

The American War of Independence ended in 1783 with a humiliating defeat for Britain. The red line was used by the British negotiating for peace with the Americans to demonstrate their interpretation of the boundary between the United States and the provinces which later formed Canada. The map was presented to George III to show him how the proposed boundaries might work.