The Blume-Leiss ALTIMeter accurately measures tree heights by obtaining the elevation angle between you and the measured points on the tree. Height measurements are read directly from the altimeter dependent upon fixed baseline distances of 15m, 20m, 30m, and 40m. Each altimeter also features percent/degree conversions and degree/slope correction factors on the back of the instrument. The Model BL8 ALTIMeter features a built-in rangefinder to determine baseline distance. A single release button and indicator make tree height measurement easy. Simply target the uppermost point of the tree and then read and record the measured value. Press the release button to free the pendulum and obtain the measurement of the lowermost point. The measured values of the uppermost and lowermost points are used to determine height.