An Abney level and clinometer, is an instrument used in surveying which consists of a fixed sighting tube, a movable spirit level that is connected to a pointing arm, and a protractor scale. An internal mirror allows the user to see the bubble in the level while sighting a distant target.
Topographic Abney Levels are used to measure degrees, percent of grade, topographic elevation, and chain correction. The user can use trigonometry to determine height & grade. 
Percentage of slope readings can also be obtained by viewing the % slope scale readings, taken from the mark on the left hand edge of the index arm.