I have been looking for a way to keep a tractor in the center of parallel rows of trees.  The current types of GPS do not work in trees.  Ultrasonic sensors are too slow to work, but laser range finders are promising.  A few weeks ago I had a light bulb moment.  Simply pointing two opposing lasers at a shallow angle to the ground might work.  I mounted a construction laser level to a quad and took it out at night to test.  It is possible to aim the laser at the base of the trees and keep the quad in the center of the trees.  The tiny red spot is hard to follow at night and can not be seen at all in daylight.  A bright 1-2 inch diameter spot should work, at least at night.  The next question is how powerful, and what color will be best?  Green seems to be the most recommended laser color for visibility.