Method of measuring the distance to horizon

[Clouds Givemethewillies]

With a photo of a suitable bit of coast with landmarks, the intersection of the horizon with the coast can be determined by the point of inflection of the edge of the sea. eg.

 

[Clouds Givemethewillies]

I also took photos of the point at Tonfanau from various known heights from nearby. Visibility was poor today..