Horizon

This is a simple app that calculates the distance to the horizon from the height of your location.

Intuitively, a person would expect that curvature of the Earth is unnoticeable in everyday life. However, it is interesting to note that after you had a walk of 5 km straight, your path actually also went "down" 2 metres because of the roundness of the planet. This can also be seen while watching distant boats using binoculars where you might notice that their bottom is "missing" below the horizon. It is well known that ancient Greek scholar Eratosthenes calculated the circumference of the Earth almost correctly based on similar curvature observations alone.

The best app usage results are of course at seashore locations, or in the large plains. Hills, mountains and other visual obstacles are not considered.

App also works excellent from an airplane. When watching the scenery from such a height, it is surprising how much one can misjudge the distance to the objects on the ground.

App can use GPS to detect height, but height accuracy of GPS seems not very high on the ground level. GPS height however works great in the airplane where height error influence is negligible.

Distance shown is the "walking" distance on the ideal round surface to the horizon. It is not the straight line from your eyes to the horizon.

DISCLAIMER: Use GPS in an airplane only with permission from the flight crew. Also, believers in flat Earth theory please be warned that this app may spoil your illusions or even break your heart.