Over time cities change, expand (shrinking is more rare,) level hills, fill in coastlines, introduce major urban elements--roads, parks and railways,--develop new street patterns and transform in all sorts of ways. Superimposing maps from different historic periods allow us to see the transformation and understand the origin and evolution of the city.
In his 1882 "Memorial History of Boston", the librarian and historian Justin Winsor published a map of Boston as it was in 1880 with the outline and topography of the original Shawmut Peninsula on top. In about 250 years, the city had completely reshaped its geography, turning what was virtually an island with an irregular perimeter and number of hills, into a regular and rather level urban mass almost three times its original area. You can see how Charles Bulfinch filled the Mill Cove with his brilliant triangle and Quincy Market established a pattern of long and narrow blocks progressively pushing out on what was the Town Cove. Atlantic Avenue and Charles Street created straight polygonal edges to the east and west respectively. The thin neck connecting the peninsula to the mainland was greatly extended on both sides with the block pattern of the South End with Washington Street as its backbone. And, of course, the largest of all the expansions, Back Bay to the southwest filling the Receiving Basin on the Charles River.