Once again, Gulliver, I am indebted to you for providing this excellent video. To my amusement I continue to be surprised by the frequency with which two people can receive exactly the same data and draw completely different conclusions.
I took several screens shots of this video and found that the horizon is in the middle of the photo and that we do not observe a declination of 5-6o as we would expect if the earth was an oblate spheroid (that model hereafter called the OSM). When the camera is pointed up the horizon curves as expected by OSM, when the camera points down the horizon curves the opposite way, and quite regularly we obtain frames of a flat horizon. We know that all camera lenses are compound spherical shapes and without making any confirming checks, I expect the Go Pro to share this characteristic. Without the optical mathematics to map pixels correctly, and without any knowledge of the plane which the camera is on, our ability to make judgements about the true shape of the earth from these images is ambiguous at best. For me, the lack of declination in many of the shots gives me sufficient encouragement to return to my original post i.e. lets perform the weather balloon experiment with camera equipped surveying equipment suitably mounted to establish the declination of the horizon. We also need to establish the height of the equipment, presumably using GPS.
With regard to Rama Set's comments, I am fully aware of the multiplicity of Geoid models, including the confusion as to whether they are gravimetric, sea level based, magnetic etc etc ad nauseum. The unambiguous point that I am making is that the are NO REAL WORLD applications of these models to any large scale mining or civil engineering projects such as mines, tunnels, railroads or bridges to compensate for the substantial differences of several metres in RL over the distances involved in these projects. Moreover, even the British Ordnance Survey continues to use the Geoid established in the 1800's rather than any later geoids because it is more accurate by a significant amount (without checking up to 80cm by memory) than the accepted international Geoid for Great Britain. My further understanding is that the application of corrections to standard civil surveys to account for the earths curvature is against the law in the UK, as discussed in Samual Rowbothams book.