When Meriwether Lewis and William Clark set off to explore the Louisiana Territory in 1804, President Thomas Jefferson directed them to note the location of interesting points along the way. The explorers diligently calculated latitude and longitude and recorded compass bearings as they traversed the continent.
But compasses have a major drawback for creating long-lasting maps. Instead of pointing to the geographic North Pole, which is constant, they align with the magnetic north, which changes as molten iron moves within Earth's core.
Lewis and Clark took sextant readings of the positions of the sun and North Star to be used with their comp