dave-in-nj:
. . .
#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
I was beginning to think that a ladder and a tape measure would be the only practical solution, but this idea makes sense. If the lasers point upwards at 45 degrees from the ground, then the laser measuring device has to me mounted 4.5 meters ( minus half the base diameter) from the tree. This ensures that the measurement is made at the required height of 4.5 meters.