Mapping Then and Now

Mapping Tools in 1803–1806

Hadley's Quadrant (also called Octant): Used for measuring the angle between the horizon and the sun (or other celestial body). Invented in 1731, it had an arc that measured 1/8th of a circle plus two mirrors that allowed the sun or another star and the horizon to be viewed simultaneously. Hadley's Quadrant can measure angles up to 90 degrees, for example, from the horizon to an object directly over head. This angle is used to determine latitude.

Sextant: Similar to Hadley's Quadrant, but with an arc measuring 1/6th of a circle. This allows the sextant to measure a greater range of angles (up to 120 degrees) than Hadley's Quadrant. A sextant can measure angles between stars, or between the moon and stars (lunar distances), which might measure more than 90 degrees. Most tables of latitude used lunar distances for reference.

Chronometer: A highly accurate clock. Accurate clocks, especially ones small enough to be portable, were not widely available until the mid-1700s. They were, however, key for calculating longitude. Lewis and Clark's chronometer was the most expensive piece of equipment in their toolkit.

The Nautical Almanac and Astronomical Ephemeris: A book containing tables of celestial observations and latitude/longitude coordinates for known locations such as observatories.

Hand Compass: A magnetic compass used to determine cardinal direction. In addition to the compass, the team took along several magnets to remagnetize the needle when necessary.

Surveyor's Compass (also called Circumferentor): Invented around 1696, a surveyor's compass differs from a hand compass in two ways. First, it includes a sight—either two bars with narrow slits or a tube—through which the target is observed. A surveyor's compass is mounted on a pole or tripod to provide a steady base. Second, because the reading is taken from the back of the needle (rather than from above, as with a hand compass), the positions of east and west are reversed.

Tape Measure: For measuring distances, a simple tape measure was used, probably made out of linen.