When measuring (chaining) up or down a slope, the angle between the slope of the chain and the horizontal is required so that the horizontal distance can be determined by calculation. This angle, measured in degrees and minutes, can be obtained using a clinometer. For small inclines, usually less than seven degrees, the correction was normally obtained from a slope correction table.
Accurately measuring long distances was an art and a science requiring corrections not only for slope if the tape (or chain) could not be held level, but also other corrections, including sag if it could not be fully supported and temperature.
Visit the article Chaining – an Art and a Science for additional information on measuring long distances.
The Society has a number of different clinometers in their collection. Three are shown here.
Clinometer, Keuffel & Essler
Clinometer, Sokkisha
Clinometer, manufacturer unknown.