A flat earther main argument is on the horizon point where remote boats are not always dissapear. However, this is related to the effective ground radious due to the spillover effect, maily due to atmospheric refraction, and the differential stratification. In this case, perhaps the earth apears dish. So what you see on the horizon is not the way to avoid any argument.
If you think the GPS and the Internet are real, then you must believe that the Earth is spherical. There is no other way for thousands of orbiting satellites to maintain their nearly circular orbits around the earth … hour after hour, day after day, month after moth, year after year. Earth satellites in orbit artificial give us the weather hemispherical image (which clearly show the curves horizons) and detailed landscape images, GPS signals that allow us to triangulate our places on the surface of the earth, and our exchange quasi -instantanée data and information with millions of users worldwide.
The forward speed of orbiting satellites to allow them to maintain their high positions on the surface of the earth as conditions change quickly are recorded continuously. (Prior to the 1960s there was no way to monitor hurricanes at sea, the sea ice drifting, or extent of flooding that the soil investigation methods can not be a member in time timely.
The problem then, is that the formulation of the question introduces a conflict of interest. You are effectively asking for a method to produce a predetermined outcome. This, in itself, would destroy the credibility of the results.
Yet even this question requires some further clauses, because ‘definitively’ requires clarification.
Many proposed methods involve using variables that are fundamentally questioned within flat earth theory.
For example … if the proposition is:
We are living in a simulation, positioned on a flat terrain, and the extra-terrestrial bodies are constructs to aid the simulation.
… the positions, or shadows, of those bodies, cannot be used to determine the outcome.
Similarly, the ‘use of a lens to determine a few pixels of curvature’ cannot be considered, because lenses typically produce similar aberrations.
Therefore, the ideal question is:
How to definitively determine whether the earth is spherical or flat, using methodology acceptable to all parties?
- Select the terrain and its dimensions carefully, to be within the capabilities of the instruments being used.
- Eliminate (or minimise to being inconsequential), temperature and humidity aberrations.
- Use variables that are standards within the civil engineering industry, primarily relating to various curvatures and refraction through air.
- Use these variables correctly (horrifyingly, most people seem to fail at basic arithmetic) – show your working out.
- In advance, determine the margins of error – they must not surpass the measurement objective (or come close).
- Survey the area, and record the reference objects.
- Determine the ‘pass / fail’ criteria.
- List all this information, detail the instruments being used, and the methodology.