I think the right question should be 'how to determine angle of flight of a butterfly from a distance with respect to the level of the ground surface'? Observer is not important for a butterfly because these insects maneuver their flight path from a distance above the ground to target the potential food plant or a resting site. To my thinking, one should take a fixed position and predetermine the perpendicular of the vegetation surface or ground surface or both. Once a flying butterfly is seen, a digital camera with facility of split-second photography should be used covering the perpendicular and flying butterfly. Now using a caliper, you can draw the vertices on a paper or on photo-print and determiner the angle of flight of the butterfly. I do not know of any device that can possibly auto-determine the angles of a flying object.

There is a new publication that provides an overview of the Pollard transect method as well as guidelines for setting up long-term monitoring programs at:

https://www.researchgate.net/publication/284721159_Guidelines_for_Standardized_Global_Butterfly_Monitoring

Technical Report Guidelines for Standardized Global Butterfly Monitoring