Length Using Monocular w/reticle and Rangefinder
The length of a limb can be measured using a moocular w/reticle and a Lase Rangefinder. The length of a limb is measured using a monocular w/reticle aligned along the orientation of the limb, the distance to either end of the limb segment, and a calculated scaling factor to determine limb length. Essentially the apparent length of the limb at each end as using the distance to that point and the scaling factor for that distance as if the limb were perpendicular to the observer. These lengths are considered to be the top and base of a regular trapezoid with a height equal to the difference in the distance between the two points. The true length of the limb can then be calculated by treating it as a diagonal of the trapezoid.
Compute limb or limb segment length using reticle and laser rangefinder
1. Assumption is that measurer cannot always get oriented 90 degrees to the axis of the limb.
2. From position of measurement (P0), measurer shoots distance to the ends of the limb, P1 and P2. These distances are the variables L1 and L2.
3. M is the reticle reading for the limb or limb segment.
4. F is the reticle factor.
5. The equations shown below are used to compute W, the length we want.
6. Note that L1 is used as an approximation of the distance of the solid brown line.
7. L2 is used to approximate the length of the solid and dashed brown line total.
8. If the ratio of the distance to the target to the length of the limb or limb segment is 9 or greater the distance error will be small.
9. Limbs over 20 feet should be divided into segments
10. The table above implements the process.
Robert T. Leverett
Co-founder and Executive Director
Native Native Tree Society
Co-founder and President
Friends of Mohawk Trail State Forest