Line of sight
Horizon, line of sight, and radio clearance.
These tools answer line-of-sight questions: how far the horizon is, how far below eye level it lies, whether two raised points can see each other over the Earth's curve, and the Fresnel zone clearance a radio link needs. Radio and antenna planners, drone operators, and mariners use them. Start with distance to the horizon. The pitfall is ignoring refraction; light and radio bend, and the answer depends on which you mean.
Planning and education aid. Not for primary navigation. Full disclaimer
I want to know how far the horizon is
- Distance to the horizon
How far you can see from a height: the horizon distance with no refraction, with optical refraction, and for radio (4/3 Earth), with the rules of thumb and their errors.
- Dip of the horizon
How far below eye level the visible horizon lies from a height, with refraction, and the navigator's 1.76′√h rule with its difference.
I want to check whether two points can see each other
- Can two points see each other?
The farthest two raised points can see each other over the Earth's curve, and for a given distance whether they can, the clearance at the midpoint, and how much of the far target is hidden.
I want to check a radio link's Fresnel clearance
- Fresnel zone and radio link clearance
The first Fresnel zone radius at a point on a radio link, the 60% clearance it needs, and the Earth's bulge there, summed into the clearance above a smooth Earth.