When Lewis and Clark became officers of the U.S. Army in the 1790s, there was no formal training in command, military history, engineering, or any other useful subjects to the military man. The United States Military Academy at West Point was established in 1802, just two years before the departure of the expedition. Some of the subjects Lewis and Clark sorely needed training in were surveying, cartography, and finding latitude and longitude. In order to make reports which would be understood by other scientific men, the explorers had to make accurate maps, plot their position on those maps, and identify the positions of natural features. William Clark may have had prior knowledge of surveying and certainly mapmaking from his previous military experience in the 1790s, or he may have learned these skills from fellow settlers in the Kentucky and Indiana region. Meriwether Lewis received his instruction in taking celestial observations from President Jefferson, astronomer and surveyor Andrew Ellicott, and mathematician Robert Patterson, who drew up a handbook of instructions for Lewis's use, the Astronomy Notebook. As far as we know, Meriwether Lewis was Clark's only instructor in the field of celestial observation. The explorers used the sun, the stars, and the moon to help plot their position at any given time.

Determining latitude and longitude was a task that occupied considerable time during the expedition. Establishing longitude required the use of a chronometer, an instrument that was in limited production in 1803 and extremely costly. A malfunction of the chronometer or, more often, neglecting to wind it affected the men's calculations, since they could not reset it with the necessary accuracy in the field. Lewis received three weeks of instruction in the use of the chronometer and in surveying from Andrew Ellicott and Robert Patterson. Even so, the accuracy of his and Clark's observations is open to question. Patterson provided Lewis with a handwritten manual for taking astronomical observations, and he and Ellicott recommended some published works that were carried with the expedition. These included Patrick Kelly's A Practical Introduction to Spherics and Nautical Astronomy ... (London, 1796); The Nautical Almanac and astronomical Ephemeris. . . (London, 1781 - 1804), which gave the daily locations of heavenly bodies, perhaps in an American edition; and Nevil Maskelyn, Tables Requisite to be Used with the Nautical Ephemeris for Finding the Latitude and Longitude at Sea (London, 1781).

Some of the tools and instruments used by Lewis and Clark included the circumferentor, with a "circle of 6 inches diameter," which was a plain surveying compass. Lewis and Clark employed it to determine bearings and courses in mapping and to find "the magnetic azimuth of the sun and pole star." The sextant was invented by Hadley and Godfrey in 1731. The sextant used by the expedition was purchased from Thomas Whitney of Philadelphia, as were many of the other scientific instruments. It was "a brass sextant of 10 inches radius." The chronometer was invented in 1735 by Harrison. Lewis purchased the "Arnold's" chronometer, of "the most improved construction," from Philadelphia watchmaker Thomas Parker, and carried it in a special case. It was of English construction; the maker is not known, but it was commonly referred to as "Arnold's" because John Arnold of London was then one of the best-known makers of chronometers. Sighting sun and horizon simultaneously through a reflecting sextant, in conjunction with the use of a chronometer, could give the explorers their position on the earth. The artificial horizon was a device that provided a reflecting surface that was always parallel to the natural horizon and was used when the latter was obscured. It was employed primarily to measure the angle of elevation of heavenly bodies. The expedition had three artificial horizons, one using water, another a glass pane mounted on a wooden ball and the third a mirror attached to a panel. A tripod stand was provided, and a spirit level was essential for adjusting the device.

What is Latitude? Latitude defines a point on the earth's surface in relation to how far north or south it lies from the equator. Latitude is measured in degrees. A point on the equator has a latitude of zero degrees. The North Pole has a latitude of 90 degrees north, while the South Pole has a latitude of 90 degrees south. Degrees of latitude are divided into 60 minutes each; each of these minutes can be divided into 60 seconds. All points on the earth's surface with the same latitude lie on an imaginary circle called a parallel. The distance between two parallels that are one degree apart is about 69 statute miles or 60 nautical miles. A difference of one minute of latitude equals about 1 nautical mile.

What is Longitude? Mapmakers think of the earth as a huge sphere divided into 360 equal parts. The lines between these parts run from the North Pole to the South Pole along the outer surface of the earth, and are called meridians. Each meridian is represented by a line of longitude on a map. Most nations start counting longitude east and west from an imaginary line, the prime meridian, which runs through Greenwich, England, a suburb of London. Greenwich lies at 0 degrees longitude. A place half way around the world from Greenwich lies at 180 degrees longitude. (The Fiji and Tonga islands, as well as Midway Island, are at roughly the 180 degree meridian in the middle of the Pacific Ocean). As another example, New York City lies at 74 degrees west longitude, or 74 degrees west of Greenwich.

The space between meridians is greatest at the equator, and narrows as the lines approach the north and south poles. For example, a degree of longitude at New Orleans is 60 miles wide. At Winnipeg, Canada, that degree is only 45 miles wide. Any point on the earth's surface passes through a circle - 360 degrees - in a 24 hour period. In one hour, 1/24, or 15 degrees of the earth passes beneath the sun. Therefore, one hour of time equals 15 degrees of longitude. Each degree of longitude is broken down into 60 parts called minutes. Each of these minutes is also divided into 60 parts called seconds. These degrees, minutes and seconds are used to measure distance rather than time, and can tell the explorer where they are in relation to Greenwich on the globe.

But time is crucial in determining longitude. As Lewis and Clark progressed westward, their chronometer could be set at Greenwich Mean Time, the point at which the sun reached its highest point over Greenwich, England. Tables in their books could also tell them the equivalent points for New York, Philadelphia, and other established cities and locations. Their position could be figured by observing the sun's position from any given point in the American West and subtracting the hours from Greenwich mean time, which they kept track of with their chronometer. Say the sun reached its zenith at the confluence of the Yellowstone and Missouri Rivers. The chronometer read 19:45, the time in Greenwich, England while Lewis stood at the mouth of the Yellowstone. There was a 7 hour 45 minute difference in time, which meant the Yellowstone River was 7¾ x 15 degrees of longitude, which made the Yellowstone 104 degrees west of Greenwich, or 30 degrees west of New York

While this subject is very technical, some terms used in Lewis and Clark's calculations may be clarified. Refraction, the bending of light rays by the atmosphere, causes a heavenly body to appear to be higher in altitude than it actually is and must be corrected for in calculation. Semidiameter must be allowed for in observations of the sun or moon; if the observation is based on the upper limb (top) or lower limb (bottom) of the body's disk, half the angular diameter of the body must be added or subtracted to obtain the position of the body's center. Parallax error is caused by the fact that the observer is standing on the earth's surface instead of its center, on which all navigational tables are based. Declination is the distance in degrees of the heavenly body north or south of the equinoctial - the celestial equator.

In their measurements, Lewis and Clark used some units of measure little known to most people today. The pole, or rod, is a unit of measurement commonly given as 16½ feet. The link is the standardized link, 7.92 inches, of the surveyor's chain made of wrought iron or steel. One chain of one hundred links measures 66 feet, or four poles, and eighty chains measures a mile. Lewis purchased a "two pole chain" (33 feet) in Philadelphia in 1803.