We were trying to figure out how high you would have to be to see the actual curvature of the earth.
Well, you would have to decide exactly at what point the human eye and mind considers that it sees a curved image.
I’ve heard that U2 pilots could see the curvature of the earth. From that I would guess that it is noticeable by around 60,000 feet. Hopefully someone else has a more solid answer for you.
One really strong joint and a big ole huff of white-out.
I’ve been able to see it from the tops of mountains.
Of course, it’s just a theory that the earth is round. 
Anyone living on the coast can see the curvature of the Earth with no problem. It’s scary. I imagined us living someplace much bigger.
If you have trouble seeing the curvature standing on a coast, try taking a picture of it. Then measure the distance from the water to the top of the picture in the middle, and then on the ends. (This is assuming you have the camera level). It is fairly apparent when you take a picture from a hotel balcony.
Make sure that in the picture you take, the horizon intersects the centre of the picture, to minimise the effect of any spherical distortion from the lens (with a wide-angle lens, it’s possible to make the horizon look concave by pointing the camera downwards.
Here is a photo taken at a south pole station:
http://www.exploratorium.edu/origins/antarctica/fieldnotes/1_3spole.html