I know you asked for a reference level, but you're not gonna get one.
Then those pictures don't prove anything.
Observations from a plane + IR already soundly debunk the globe, because they don't match globe model predictions at all like I said. So far all the globe zealots have ignored that information, including you. Why?
Do tell. What does the globe model predict that the pictures from a plane + IR predict that the horizon should look like?
Thanks for confirming that the horizon is a result of perspective and not "curvature of Earth". 
The horizon is a result of earth and sky appearing to meet in the distance and will exist regardless of the shape of the earth. The salient question is whether or not a object can move past and dip below the horizon. On a flat earth, the answer is no. On a round earth, the answer is yes.