Expedition to Peru measured the curve of the earth.

They had problems.

They had that guy who used expedition money to buy diamond ring for his girl before he was kicked out.

They had to wait many times till bad wether is over.

But they did it.

They even measured the small difference between curvatures south and north of the equator.

Flat Earther sits at home, read Rowbotham's scam book and hope they can prove and spread his deceptions.

Hard to admit they were seduced by lies of one charlatan.

Unfortunately, "everyone" can use theodolite and measure how wrong they are when sayin the earth is flat.

When I say "everyone", I mean it is not limited to very small, rich elite group.

If you have a job, you can buy theodolite and see the reality. It is cheaper than iPhone X.

For example:

Theodolite on eBay.

Yes, you would have to understand geometry and learn how to use the device.

But there are articles about it all over the Internet. Easy to google for them.

Two verticals at the distance of one nautical mile will not be parallel.

They will tilt from each other by one arc minute.

That was how and why the nautical mile was defined in the first place.

One degree is 60 nautical miles wide.

(Not "one degree" between meridians. One degree on local great circle.)

Flat Earther will say "it is not true, definition of nautical mile is conspiracy". (Try to tell that to people who defined it because they needed it.)

In reality, nautical mile was defined long before NASA.

Difference between Euclidean and non-Euclidean geometry is, for example, diference in sum of angles in triangle.

Euclidean geometry works on flat plane, and sum of angles in triangle is always 180 degrees.

Non-Euclidean geometry works on curved surfaces.

On the Earth's surface if you measure triangle big enough, sum of the angles will be more than 180.

If the Earth was flat it wouldn't be the case.

Bigger the triangle, more degrees in the angle sum.

If you draw a triangle with one point at the South Pole, draw two sides with angle between them 90 degrees,

and draw them all the way to equator, they will intersect it under another 90 degrees angles.

Such triangle has sum of angles 270 degrees.

All three sides of the triangle will follow great circles of the globe, which means they will not turn left or right, only down, following the curve of the ground.

You can use theodolite in several ways, and if you understand geometry you can measure curvature yourself.

Flat Earther will avoid it. He/she will grasp every excuse to discard, even discredit your measurements. And hide behind cherry-picked evidence (or "evidence").

But to honestly measure on their own?

No way, Jose.

EDIT: I didn't have to buy one. I knew people in and around construction business.

Some of them were geodesists. (Especially important for building roads, but you already knew that.)