Equipment

http://www.nps.gov/jeff/LewisClark2/TheJourney/mapmaking.htm
http://www.nps.gov/jeff/LewisClark2/TheJourney/More%20Information%20on%20Surveying.htm
Equipment
Captain Lewis indicated in his lists of expedition equipment that they carried quite a few surveying instruments, but all of them were far more useful for short rather than long-distance measuring. These included a "two pole chain" (33 feet long), a "log line reel" which measured the rate of boat travel (these measurements were affected by river currents), compasses, quadrants, sextants, and a chronometer. Early in the second year, the chronometer ran down, which further complicated things by making it impossible to determine longitude. Routine measuring was likely estimated or gauged by 'dead reckoning.' The fact that various journals give differing mileage figures for the same area traveled supports this idea. The mileage estimates are of little help in determining specific geographic points or campsites. Journal entries and geographic landmarks on the maps are more reliable guides. Clark probably based his mileage figures on the time of travel or his skill and experience as an outdoorsman. Clark relied on compass readings for his "courses." His compass notes are at the beginning of nearly every journal entry. He probably employed the route traverse method, taking bearings at each turn of the trail or bend in the river and plotting those shifts on his maps.... For his mapping Clark was probably taking 'back sightings' or 'back azimuths,' to get his bearing from north or south in degrees. After traveling forward some miles, Clark would turn and take an azimuth reading from his previous point of sighting. This procedure was necessary because he could not always determine his next point of observation. In converting back azimuths to forward readings, Clark may have become confused occasionally, and some of his readings may seem turned around because they reflect the direction he was looking rather than the direction that he had traveled."