While looking at David Hogg's YT channel after reading about the shooting, I found this:
A docuмentary about the students at Parkland launching a high altitude weather balloon.
Predictably, you will see the rate of curvature bend back & forth in unison with the camera
swinging.
When the camera is still the horizon is totally flat, of course.
Go to 15:00 mark to skip the preamble.
Here's a still screencap:
https://imgur.com/GpOcflYNo NASA needed.
Civilian balloon at 100K feet shows a flat earth.