English: The confusing routes of several explorations led by U.S. Army Engineer Major Stephen H. Long appear on his map that was first engraved and printed for Henry Carey and Isaac Lea's atlas, published in Philadelphia in 1822. In the first of these expeditions, begun in the summer of 1816, Long made a survey of the Illinois River and its tributaries to Fort Dearborn, eventually the site of Chicago on Lake Michigan. Shown on the map along the Illinois River and at the southern end of Lake Michigan are multiple inscriptions reading "Long's route 1816" and, farther west, a "Proposed National Road" – an idea Long suggested along with canals to connect the big lake with the river in his official report.

In the following years Long explored the Mississippi River to the Falls of St. Anthony (the future site of Minneapolis with the trail denoted as "Long’s route 1817"). Long went down the Mississippi River and then up the Arkansas where he helped construct Fort Smith. He soon inspected the Southwest border with New Spain to the Red River beyond Pecan Point (as had the Freeman-Custis expedition) and then returned overland (denoted as "Long's route Dec. 1817") back to the Arkansas. He next passed through the Arkansas Territory and southeastern Missouri to the Mississippi River again ("Long's route 1818").

In the fall of 1818, Long headed back east to get married and also began construction of a new type of shallow-draft steamboat that would accompany a military and scientific expedition up the Missouri to the Yellowstone River. Long's steamboat The Western Engineer went into operation in 1819 to navigate the Ohio, Mississippi, and Missouri Rivers. His expedition established a winter quarters at "Engineer Cantonment" – located along the Missouri south of Council Bluffs.

As a result of the 1819 Adams-Onís Treaty concluded with Spain, Long next set out with new orders to explore west to the headwaters of the Platte, Arkansas, and Red Rivers in order to dispel confusion about the new boundaries between the U.S. and New Spain. Long and his men reached the Rocky Mountains that summer and then split into two groups, one under Captain John R. Bell to follow the Arkansas back eastward and another led by Long himself to follow the Red River. Unfortunately, Long discovered too late that the river he thought was the Red was actually another, the Canadian.

In 1821, Long, having recognized his mistake, produced a large manuscript map employed as the basis for the map in the Carey & Lea Atlas that noted the correct approximate position of the Red River and the boundary. In addition to showing correct state and territorial boundary configurations of the time, the map, most significantly, helped popularize the idea that the American West was an inhospitable desert filled with hostile Indians by labeling the western portion as a "Great Desert" and noting it was "frequented by roving bands of Indians who have no fixed places of residence but roam from place to place in quest of game."