I saw the sign;
"Flat Earth Lake Test", 3pm, 2 Feb, Alton town docks. There is a lot on the net such as:
https://www.youtube.com/watch?v=0hv127-2vI8
IMHO it is very difficult to accuracy conduct such a test such as taking into consideration atmospheric refection at that instant in time and just above the ice/snow surface. On winter days there is normally a temperature inversion just above the ice surface which can result in refracting the laser light to follow the curvature of the several mile ice surface.