Have you ever been at altitude and looked at the horizon
wardaddy,
Yes. Many times. Hundreds of times.
It is as flat as a pancake as far as the eye can see.
More, the vanishing point (”horizon”) rises to meet the eye. That is only possible on a flat earth. On a “globe” earth, the “horizon” would drop away and you would have to look down more and more to keep your eye on it as you increased your altitude.