Theodolites are precise instruments used by surveyors and other professionals to measure angles, both vertical and horizontal. A very simple task for a Theodolite is to measure the angular dip of the horizon. Theodolites are extremely sensitive and are certified to measure angles to 1 arcminute or less. That is 0.00028 degrees.
These measurements were take at Williams Reset by The Main Surveyor, a professional geodetic surveyor.
Coordinates: 43° 37′ 26.52402″ N, 070° 12′ 37.43712″ W
Elevation: 21.986 meters
Note that there are two views, one at 90 degrees, one at 270 degrees. These two measurements are to negate the effect of any collimation errors. The final image has field notes and calculations from The Maine Surveyor. A total of 20 measurements were taken, 10 for each face of the theodolite. By averaging equal numbers of measurements from each face collimation errors are eliminated.
The key thing to identify here is that the horizon does, indeed drop when measured. The measured drop is 7′ 39″ with a standard deviation of +/- 3 arcseconds.
You can watch the video by The Maine Surveyor here.