When the trunk base is above eye level, the base reading is subtracted from the apex reading, and the total height is the difference percentage of the horizontal distance.
For example, if the apex reading is 65 % and the base reading 14 %, the total height is (64 – 14) / 100 x 25 m = 50 / 100 x 25 m = 12.5 m or equally in feet (64 – 14) / 100 x 82 ft = 50 / 100 x 82 ft = 41 ft. When calculations are made mentally it is advisable to use measuring distance of 50, 100 or 200 m / ft. for the sake of simplicity.
All readings of the percentage scale are based on the horizontal distance. This means that if the distance on sloping terrain is measured along the ground an error is introduced, and this must be corrected for accurate results. The error is insignificant for most purposes at small ground slope angles but increases progressively as the angle increases.
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