The idea that the Earth is spherical is supported by overwhelming scientific evidence and observations from various fields such as geodesy, satellite imagery, and space exploration. However, let's address your specific question regarding the visibility of curvature at a height of 54 km.
At an altitude of 54 km (33.6 miles), one would still be within the Earth's atmosphere, specifically in the upper region known as the stratosphere. While the curvature of the Earth is indeed observable from higher altitudes and in space, it may not be immediately apparent at 54 km due to the large scale of the Earth compared to our visual perception.
The perception of "flatness" at that altitude is primarily influenced by the vastness of the Earth's surface relative to the observer's field of view. The curvature of the Earth is gradual, and from that height, the observer's line of sight covers only a small portion of the planet's surface, resulting in the appearance of a flat horizon.
However, as an observer ascends higher, such as in a high-altitude balloon or an aircraft, the curvature becomes increasingly evident. Astronauts in orbit or aboard the International Space Station (ISS) have a more comprehensive view of the Earth's curvature due to their significantly higher altitude.
Moreover, there are various ways to demonstrate the Earth's curvature at lower altitudes. For instance, long-distance observations over bodies of water, such as seeing ships disappear hull-first beyond the horizon or observing the dip of the horizon from elevated vantage points, provide evidence for the Earth's spherical shape.
It's important to note that the perception of a flat horizon at 54 km does not invalidate the overwhelming evidence and scientific consensus supporting the spherical shape of the Earth. It simply highlights the limitations of human perception and the need to consider a broader range of observations and data to understand the Earth's true shape.