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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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