This official handbook of the U.S. Forest Service explains how to use the Abney hand level, a clinometer tool used to measure angles in various natural environments. The Abney tool was invented by William de Wiveleslie Abney during the 1870s. By the time the device was recognized and widely-used in the early 20th century, it had been refined. Though earlier guides cover its basic usage, this U.S. government commissioned handbook offers demonstration and instruction on the tool's abilities. Hartley Calkins and Y. B. Yule were themselves experienced surveyors with experience traversing and measuring the rugged woodlands of North America. The Abney level is used to this day by surveyors, geologists, foresters, and safety inspectors; in good hands, it is reliable for its accuracy. At roughly $60-100 in price it is affordable, durable, lightweight and portable. The Abney Level Handbook, first published in 1927 and expanded in 1935, remains the authoritative source as it covers the subject superbly.