Get yourself a 10 metre length of string, lay it out in a circle and you will see it curved.
Now get a magnifying glass and view one piece of the curved string and hey presto, you will see a straight edge, a flat edge.
15 km divided by earths circumference of 40075 km is 0.0374% which is much smaller than the percentage of straight string you see with the magnifying glass, so you will not observe the curvature of the earth at such a small distance! The video starts off with a lie and goes STRAIGHT downhill after that.
I would suggest going into space for proper observation of curvature but those lying NASA guys have photo-shopped everything so there is no space! LOL.
No PhD required, just common sense.