## Force the Line

“Force the line” is a method proposed by flat-Earthers to determine Earth’s shape. It is an unrealistic method involving building a very long structure that is maintained flat for its entire length.

“Force the line” is just an exercise of one single proof fallacy. They ignore the overwhelming amount of evidence in support of spherical Earth, withholding their judgment for that one specific method, which is very difficult to execute and prone to errors.

## Danyang–Kunshan Grand Bridge

As of 2021, the Danyang–Kunshan Grand Bridge is considered the longest bridge in the world. It is a 164.8-km-long viaduct bridge in China, consisting of 2000 spans of ~80 m long each. Like every other bridge, it is constructed following Earth’s curvature.

A hoax in flat Earth communities claims the Danyang–Kunshan Grand Bridge somehow “proves” flat Earth. In reality, it is just a claim asserted without evidence and then spread as a hoax.

The bridge is a viaduct with many short spans, and therefore, accounting for Earth’s curve is less critical than in the case of longer-spanning bridges. Nevertheless, it is still built to follow Earth’s curvature.

Railways are built to follow Earth’s terrain, minimizing the grade/slope as much as possible. Consequently, they all follow Earth’s curvature. And many of them were built by explicitly accounting for the curve of the Earth.

Flat-Earthers claim that railroads are straight, and never built by accounting for Earth’s curvature. They are wrong. The leveling works were done in such a way to minimize the effect of Earth’s curvature. There are also multiple books describing railroad constructions where the curvature of the Earth is specifically accounted for.

## Leveling

Leveling is the process of determining the elevation of a point relative to another point. The curvature of the Earth and atmospheric refraction affect the result of leveling. There are techniques and formulas to correct the effect of Earth’s curvature and atmospheric refraction.

Flat-Earthers assume that construction works like roads, bridges, railways, etc. are built without accounting for Earth’s curvature. They are wrong. The leveling in such works are done in such a way it minimizes the errors due to Earth’s curvature and atmospheric refraction.

## Humber Bridge and Earth’s Curvature

The Humber Bridge, near Kingston upon Hull, East Riding of Yorkshire, England, is a 2.22-kilometer (1.38 mi) single-span road suspension bridge, which opened to traffic on 24 June 1981. When it opened, the bridge was the longest of its type in the world.

The towers, although both vertical, are 36 mm (1.4 inches) farther apart at the top than the bottom due to the curvature of the earth.

## The Verrazano-Narrows Bridge and The Curvature of The Earth

Accounting for the curvature of the Earth is not usually needed for narrow high-rise building projects. Designers only need to ensure the foundation is flat, and the curvature of the Earth becomes non-factor.

For projects that extend over a long distance, like roads, railroads, canals, etc., they are built along the curvature of the Earth, and specifically accounting for the curvature is usually not needed.

But when the project extends on a long distance, as well as extending upwards, then we have no choice but to take the curvature of the Earth into account. One of such projects is the Verrazano-Narrows Bridge, New York, United States.