earth curvature

An important clarification before we begin: my idea here is not to debunk the author's entire line of thought (which I of course disagree with), but rather to refer to a very specific statement made in the following video: Source: https://youtu.be/v0udDmzdERM (The claim starts aprox at 08:39)...

Source: https://m.youtube.com/watch?v=nCaGCUy6XMk Source: http://imgur.com/oqIx5oM The video shows a portion of the Isle of Man from the waterline of a beach in Blackpool, at a distance of roughly 65 miles, and claims that this should be impossible on a globe, therefore the earth is flat...

This is Folsom Lake, California. I visited it today (Aug 1 2016) to try to determine the curvature of the Earth. Most people know the Earth is round, but they know it mostly because they just accept it, as it's long established science. But you can actually go out and test this for yourself...

I keep being presented with 'earth curvature experiment' videos recently, by flat/concave earth advocates. It seems to be their new favorite "evidence" that Earth is not spherical. Debunking this gets into math which I stink at, regarding refraction. I would like to be sure I have considered...