I think the problem is more simple.

you go into a forest and want to cut only trees that are 1 meter or larger in diameter.

you take your harvester to a tree,

from 4.12 meters away, sensed by your range finder

and at 4.5 feet from the ground

the visual angle of the tree is 2.31 degrees.

now you have all you need to calculate the unknown side. the diameter of the tree.

it it is larger than spec, it becomes Mrs Finnegans new chest of drawers and lumber for the new house.

by feeding the vision input into your R-Pi, you can do the math

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#2

you approach a tree, slip your verniers over the tree and by holding them 4.5 feet above the ground as sensed by your range finder, you move them till they touch the tree on both sides. then measure the distance on your verniers based on movement of the verniers.

#3

you have your two lasers, each mounted parallel to each other,

on a rail that allows you to separate them.

you move to your tree, then slide them so that each one touches the side of the tree.

your ir sensor is to 'see' that it touches the tree and reflects or not...

measure the distance of your lasers, that equal the tree diameter