Hi Giles, Two Martinis later - stirred, not shaken - let me try and clear up the problem. First the boat will have to be place on a flat surface and adjusted so that the water line is parallel with the surface. You then also have to make sure that the boat is level - Port to Starboard - Now you have to set up your laser so that the laser is parallel to the floor. Then you have to adjust the height of the laser (always keeping it parallel to the floor) so that it hits the intended water line (a vertical adjustment). Now, if you sweep the laser from bow to stern, it should define the waterline. It is determining a plane. and that plane is the waterline. Hull curvature doesn't enter into it, since the laser plane will cut the hull to match your adjusted setup height of the laser. You are right, if you don't have the laser in the same plane as the surface that the boat is sitting on. Then you can probably draw waterlines that would put Picasso to shame. .............but the Navy does that when they camouflage paint their vessels. If someone objects, say you are the Navy Auxiliary! Once the snow is gone, and it's time to do boat work again, I'll report on the ease or difficulty I had with my intended "solution" Connie