Elevation Model and Satellite Image from September 14, 2000
Courtesy of Jeff Silkwood, EOS Education Project, University of Montana
Compare the above image with the map Clark drew from Sheheke’s information, and the Warren map of 1855, showing the "War Path of the Big Bellies."
resident Jefferson's orders to Lewis were expressly to take careful observations of latitude and longitude, especially at the mouths of rivers. Those readings were to serve as checks on the daily compass courses and estimated distances between landmarks.1 Nonetheless, Clark continued to record courses and distances, although the latter were much less accurate than his measurements on the Missouri. For example, he estimated the length of the Yellowstone from the Big Bend where he reached it, to its confluence with the Missouri, at 837 miles (1347 km). Today the same stretch measures 498.2 miles (802 km), and its channel has not changed appreciably during the past 200 years.2 But Clark did not make any celestial observations on his entire journey down the Yellowstone, and even if he had, he would not have had the knowledge of trigonometry to complete the calculations. So he could only guess at the distance Pryor would need to cover.
In 21st-century terms, the confluence of the Bighorn River with the Yellowstone is located at approximately 46 degrees, 10 minutes north latitude and 107° 28' west longitude; the mouth of the Knife River on the Missouri is at about 47° 20' north latitude and 101° 23' west longitude. In other words, the mouth of the Bighorn is 46° 10' north of the Equator, and 107° 28' west of the prime meridian, which runs through Greenwich (pron. GREN-itch), England. And so on. (The lines marking degrees of latitude on a map are called parallels; lines indicating longitude are called meridians.)
A degree of latitude at about halfway between the equator and the North Pole is equal to approximately 69 miles (111 km); a minute is 1/60th of that, or 1.15 miles (1.85 km). A degree of longitude at the same parallel equals 48.35 miles (78 km), and a minute, 0.8 mile (1.3 km). We know, therefore, that the extremities of Sheheke’s route lie on meridians that are 249.1 miles (401.7 km) apart, and on parallels that are 80.5 miles (129.8 km) apart. Therefore, if Sheheke's route really was a more or less straight line, it would have been 329.5 miles (531.5 km) long.