Borderline trees occur only when the distance from the sampling point to the center of the tree is equal to the DBH times plot radius factor (PRF).
The PRF is determined based on the type of prism or angle gauge being used.
Basal Area Factor (BAF) 5, 10, and 20 angle gauges result in PRFs of 3.89, 2.75, and 1.94 (feet inch−1) respectively.
A tree that appears borderline using a BAF 10 instrument was measured as 12.4 inches (310 mm) DBH.
The horizontal distance from the sampling point to the center of the tree is 34 feet (10 m).