Why is this "independently reproducible proof of non-curvature" wrong?
The biggest flaw I see in a cursory look at this work is failure to compensate for atmospheric refraction.  Density varies in the atmosphere enough that the refractive index varies.  At low angles above the horizon the temperature structure of the atmosphere becomes very important in determining optical refraction and at low angles very close to the ground it becomes extremely important.  

When cool air is at the surface and warms with height (e.g. a temperature inversion), which is likely above a cool body of water like a canal (or in winter, or at night and early morning), the resulting density gradient causes light to bend toward the ground.  This means whatever you are observing is actually lower than you perceive it. 


http://images.tutorvista.com/content/light-refraction/looming.jpeg  

In the situation where it is warm near the surface and cools aloft you get the opposite and light bends up toward the sky.  This gives rise to the water mirages you can see on hot days on roads or sand (where there is a very strong temperature gradient with height).