First of all. I know earth is a sphere. I got the idea that since we with a theodolite can see earth curve directly, I think a simple experiment could be used to more specifically prove the famous drop. A theodolite set up exactly level, A drone flies out over the ocean keeping in the center of the crosshair at all times. Or at least when taking readings. A method for a drone to measure the exact distance from itself down to the water below. My initial idea would be a laser range finder. But I'm not sure if that would work on water Or would an ultra sonic work better? Is there any known way to accurately determine distance to something bouncing it off water Or will a simple altimeter using air pressure be accurate enough ?