The tour uses Jim Duffin’s layered mapping tool as the base map to show the historic water coverage over the current area of the Navy Yard. Before aerial photography and satellite technologies standardized mapping, differences in hand drawn historic maps were as likely a result of measurement errors as actual changes to the area. Duffin’s method of georeferencing scans of maps and atlases from the eighteenth and nineteenth century plots historic boundaries within the modern landscape as accurately as possible. Through the nineteenth century, the “boundaries” of League Island shifted daily with the tide, which many maps represent using high and low water markers or areas of wetlands. Still, this map provides a general sense of League Island’s size in the early nineteenth century in comparison to the Navy Yard campus as it exists today, solidified from dredged river mud and concrete.