The John Melish Map of the United States

The John Melish map juxtaposed for the first time the expeditions of

1.  Lewis and Clark up the Missouri River in 1806-7

2. Zebulon Pike in the American Southwest from 1806-7

3. Stephen Long in the American West from 1819-20

4. Andrew Ellicott on the Spanish American Border from 1796 to 1800

5. Thomas Nuttal up the Missouri River and in Arkansas in 1811 and
from 1818 to 1820.

When this map was published in 1816 (later editions came out up to 1824)  - the first one to show a United
State with the possibility of reaching from the Atlantic to the
Pacific Ocean, the idea of "Manifest Destiny" FIRST came into the minds
of Americans then all living on the East Coast.  As second and third
sons with no inherited land saw this map, they saw the vastness of
America and decided to head West.  This map is one of the primary
reasons for this event.

It is by far the most important map of America published in the 19th century.