Earth’s shape has been measured many times.  It’s curvy.

If the earth is a sphere then the horizon must drop from the apparent level, sometimes called “eye level”.

This is predicted to be very slight so any measurements must be capable of more precision than the predicted dip.

• Horizon dip measurement from Maine
• Horizon dip measured from New Zealand
• Detailed and well-controlled experiment measuring dip with increasing distance

If the earth is a sphere then the horizon must curve left to right.

This is predicted to be very slight so any measurements must be capable of more precision than the predicted curve.

• Left to right curve rocket at nearly 300,000 feet
• Left to right curve of the horizon from Ibiza
• Left to right curve of the horizon from Norway
• Dwayne Kellum’s high altitude balloon
• First photograph of the curve of the horizon

If the earth is a sphere vertical lines must diverge.

This is predicted to be very slight so any measurements must be capable of more precision than the predicted divergence.

• The Maine Surveyor, a licensed geodetic surveyor used two theodolites to precisely test for the divergence of vertical lines.
  • Are Plumb Lines Parallel? – Geodesy
  • Plumb Lines – Still Not Parallel
  • Plumb Lines – Follow-Up 1
  • Plumb Lines – Follow-Up 2
  • Plumb Lines – Follow-Up 3
• Jesse Kozlowski, a licensed geodetic surveyor measured the divergence of plumb lines over one mile.
  • https://jessekozlowski.wordpress.com/2016/11/24/one-mile-level-line/
• Jesse Kozlowski also measured the vertical drop over a still and level lake.
  • https://jessekozlowski.wordpress.com/2016/11/05/looks-perfectly-flat-and-level-to-me/

“Looks flat to me”

Our eyes are not sensitive measuring devices.  We are incapable of distinguishing 1 degree of arc from a straight line.