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

If the earth is a sphere there should be photos of the curve.

Enjoy these photos of the curve

If the earth is a sphere it must curve away from us.

The Bonneville salt flats show this nicely

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. Typically 40,000 feet is high enough to photograph with a camera and see it on the photo with our own eyes.

• Wolfie6020 video of the curve from 46,000 feet
• 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 then measuring to a distant point that is higher in elevation is will be below horizontal

• On a mountain 6,888 feet in elevation measuring to a mountain 10,709 feet that is 114 miles away it is BELOW horizontal from the lower mountain. https://www.tiktok.com/@baronrutledge/video/7443044365661965599?is_from_webapp=1&sender_device=pc

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

The “dip from eye level” for flat earth is very close to zero. The dip for the globe increases with elevation so this is a good test provided the instrument is can measure the dip with enough precision. From the ground a professional Theodolite is well suited to measure the dip:

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

From the elevation of an airplane at 35,000 feet the predicted globe dip is 3.2-3.5 degrees, the flat earth prediction for the same height at 40°N Latitude is 0.0239-0.0423 degrees. Smart phones can measure to the nearest 0.1 degree so are a great way for anyone to test this, here are several measurements:

• Horizon does not rise to eye level

If the earth is a sphere vertical lines must diverge: Reciprocal Zenith Angles.

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

• Baron Rutledge, measured reciprocal zenith angles in a video, plumb lines diverge.
  • Reciprocal Zenith Angles – Baron Rutledge
• The Maine Surveyor, a licensed geodetic surveyor used two stations 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/2016/10/27/flat-level-lake-measurments/
  • He repeated it for a longer distance:
  • https://jessekozlowski.wordpress.com/2016/11/05/looks-perfectly-flat-and-level-to-me/

If the earth is a sphere large triangles must have more than 180°.

A spherical triangle is a triangle on the surface of a sphere. These are not to be confused with a “planar triangle” which is what we deal with regularly. The amount that a spherical triangle sums to over 180° is called “spherical excess”. This is a well understood topic that was written about by Euclid millennia ago. Wolfram has excellent information on spherical triangles.

Here is a video I made demonstrating it: https://youtu.be/pJ9xW-nYCNM

Spherical triangles have been measured precisely for a long time: Spherical Excess

“Looks flat to me”.

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