In the midst of this chopping sea of civilized life, such are the clouds and storms and quicksands and thousand-and-one items to be allowed for, that a man has to live, if he would not founder and go to the bottom and not make his port at all, by dead reckoning, and he must be a great calculator indeed who succeeds. – Henry David Thoreau
For most of us, who grew up with Rand McNally road atlases, DeLorme Gazetteers, and now GPS, it’s pretty hard to imagine being launched into completely unknown territory without a map. How in the world would you know where to go? It’s even harder to imagine being the one asked to make the map from scratch. How did Lewis & Clark do it? Where would you even start?
William Clark was the expedition’s cartographer. It’s fun to laugh about Clark’s spelling; it really is hilarious. But it also sometimes makes it easy to forget just how smart Clark was. His maps of the American West are one of the great achievements in the history of cartography, and one of the most enduring legacies of the Lewis & Clark Expedition.
Meriwether Lewis was given virtually a blank check to outfit the Expedition, and he used a fair chunk of it in preparation for the mission’s mapmaking. He researched and bought the finest scientific navigational instruments available in Philadelphia, including sextants and octants that could help measure angles to the sun and determine latitude; high-end compasses to assist with measuring angles to various stars; and a fancy watch called a chronometer to help determine longitude . (In fact, at $250, the chronometer was the single most expensive piece of equipment taken on the Expedition). He also bought surveyor’s chains, pocket compasses, artificial horizons (for use in the mountains), and the latest almanacs and mathematical tables. Morever, Lewis underwent a crash course in how to use the instruments, knowledge that he later passed on to Clark.
Though the instruments were break-through technology in their day, and would eventually evolve into the kinds of amazing navigational devices we have today, at the time they were cranky, difficult to use, error-prone, and required time, patience, and big-time math skills to operate. Though Lewis and Clark faithfully tried to use them, in practice Clark, who soon took over almost all responsibility for mapping and navigation, relied heavily on a traditional process called dead reckoning.
Dead reckoning is simple in concept, but requires a disciplined mathematical mind and the ability to grasp both details and the big picture. The basics are these: to make a map of unknown territory, you first start with the last known point. Then you use a calculation to track your progress. In short, if you know your rate of speed and how long you have traveled, you can calculate distance.
Clark knew how to employ dead reckoning both on foot and from the river. Since the Expedition usually needed to be on the move, most of Clark’s work was done from a boat, though he did use the surveyor’s chains on land when he wanted to calculate the width of the river from various points. From the boat, Clark used a compass, a watch, and a log line. When he sighted a handy landmark, such as a creek mouth or an island, Clark would note the exact time, take a compass reading to find the landmark’s direction, and then throw a weighted piece of wood over the side. The weight is on a rope marked with knots and loops, and as the boat travels, you can count the number of knots that pass by in thirty seconds. This is how you determine how fast the boat is moving in nautical miles or knots.
Clark repeated this process for each landmark, noting the section measured, the direction traveled, and the time elapsed. Here is an example, from Clark’s journal of June 6, 1804:
|N. 28° W||3 ½||ms. to a Hill on S. S. pd. N. Bilg: of Isd.|
|N 49° W||1 ½||Ms. to a Creek Split rock|
|West—||1 ½||Ms. to a pt. on S. S. opsd. a Clift|
|N 31° W||4 ½||ms. to a pt. on L. S. psd. Saline C. L. S.|
|N. 51° W||3||ms. to a bilg of an Isd. to left pd. Sm. Isd.|
It’s fun to contrast this with Clark’s journal of the same day and see what details he recorded that might have helped him later with his map:
Mended our Mast this morning and Set out at 7 oClock, under a Jentle Braise from the S, E by S N 28° W 3½ miles to a hill on St Sd. pass:g the N: belge of the Island Called Split rock Island, the river rose last night a foot the Countrey about this Isd. is delightfull large rush bottom of rushes below on the St. Side N 49° W, 1½ Ms. to the mouth of Split rock River blank yds. wide on the Starboard Side opod. the pt. of a Isd: passed a place in the projecting rock Called the hole thro’ the rock, a round Cave pass thro the Pt. of rock’s
West1½ ms. to a pt. on Std. Sd. opposit a Clift of rocks abt 200 foot N 31° W. 4 ms ½ to a pt. on L: Side passed Saline Creek on the L. Side a large Salt Lick & Spring 9 me. up the Creek, one bushel of water will make 7 lb. of good Salt
(Information) Took Meridian altitude of Sun Limb. 37° 6′ 0″ equat to blank of Lattidude.
on this Creek [Saline], So great a no of Salt Springs are on it that the water is brackish N 51° W to a Belge of an Isd on the S. Sd. at 3 ms. Passed a Willow Isd. in Middle, Some wind in the after part of to day from the S E, (the Banks are falling in greatly in this part of the river) as also is one Side or the other in all the Course, we assended on the North Side of the Isd. and finding that the perogues Could not Keep up Camped 2 hs. by Sun. on the Sd Sd
the land below this is good.
Every few days, Clark reconstructed his notes into a grid map of the river to scale. Historians believe that Clark probably kicked around his observations with Lewis and some of the other men, especially the sergeants who were specially assigned to act as lookouts and observers along the river.
It seems like magic to me, and as you might expect, whether a map produced this way was accurate depended entirely on the skill and good judgement of the observer. Fortunately for Lewis & Clark’s historic legacy, it turned out that William Clark was a dead reckoning genius.
A disadvantage of dead reckoning is that since new positions are calculated solely from previous positions, the errors of the process are cumulative, so the error in the position fix grows with time. So how did Captain Clark fare? When the Corps of Discovery arrived at the Pacific Ocean in November of 1805, Clark estimated that they had traveled 4162 miles since leaving Camp Wood some 18 months earlier. He was off by a mere 40 miles.
More great reading:
The Spatial Legacy of Lewis and Clark (Clark’s maps compared with satellite photos from space)