This detail from Clark's map of 1814 shows his post-expeditionary conclusions regarding the lay of the land from just west of the Three Forks of the Missouri River, roughly 230 air miles eastward along the Yellowstone to the Tongue River. It shows how much Clark learned from the Indians, and how much he learned on the ground himself. It also reflects a little of the information he got from former expedition members like John Colter, as well as other travelers to this part of the West between 1806 and 1813.
Bear in mind that Clark knew the approximate latitude of the Three Forks, as well as the confluence of the Yellowstone with the Missouri, but he didn't take any astronomical readings during his trip between those points, and he had nothing more than a feel for the longitude of either. He kept daily records of compass bearings and estimated distances between landmarks along the way, but his distances were overall so far off that he concluded the length of the Yellowstone from the Big Bend to the Missouri was 837 miles, when it was probably about what it is today, 438 miles.
For all its "mistakes," by 21st-century standards, it also shows Clark's extraordinary ability to observe and remember details, and to extrapolate from them generalizations about the terrain he covered. In other words, Clark had a capacity for geographic memory and imagination that enabled him to feel the earth beneath and around him. In comparison with the map he drew in 1810 (not shown here), which was a preliminary compilation of his field sketches, the 1814 map also shows how his grasp of Northwestern geography matured over time. With it he gave a palpable shape to the land he had traveled, which other travelers and explorers were to rely upon for the next fifty years.