Summing the interior angles of a triangle on a flat surface will always be 180 degrees. The sum of the interior angles of a triangle on the surface of a sphere must be greater than 180 degrees and less than 540 degrees.

Pilots have successfully used Operational Navigation Charts (ONC) for decades. These maps work, if they didn't, flights would get lost, run out of fuel, etc. Planes would have crashed in large numbers, the people that own the planes would stop using faulty charts to protect their costly assets.

I used paper ONC charts to plot out a route around three airports, then pieced together the paper charts. These airports have regular weekly flights in each direction. The great circle routes form a spherical triangle, the sum of the interior angles of this triangle is 263 degrees. A complete impossibility on a flat earth. The projected great circle distances for each leg match the time taken for each flight based on the air speed for the airplanes flying each flight.

Here is the video documenting this:

I also redid the route for arbitrary points on the globe, not associated to airports to accurately meet the exact requirements of a challenge of exactly 90 degrees at each turn:

This entry satisfied the challenge and was declared to be the winning entry, though the challenge issuer welched and claimed the challenge had been closed. He had clearly stated numerous times that in-progress entries would be accepted. No matter, as stated in the video, I was not going to accept any of the money, half to charity, half to be held for winnings in future challenges.

So, as ONC charts must be accurate and have not, to my knowledge, been proven to have errors, this is solid evidence of a spherical earth.