Line of sight calculator – can A see B over the terrain?

Line of sight decides whether a radio link works, whether a wind turbine will be visible from a village, whether a drone stays in view of its pilot and whether you can see one summit from another. The question is always the same: does the straight line from one point to the other pass above every hill, ridge and roof between them? Click on the map to place point A, then point B, and set how high above the ground each end is: eye height for a person, the height of an antenna on a mast, a drone's altitude or the hub of a turbine. The tool reads the ground height at 400 points along the path, adds the curvature of the Earth, and draws the side view: the terrain, the straight sight line and, if you enter a radio frequency, the first Fresnel zone around it. The line on the map turns red from the first point where the terrain gets in the way, and the panel says how far from A that is, how far the ground rises above the line, and the lowest height at A or at B that would give a clear view. The share link stores both points and all settings, and the path downloads as CSV, KML or GeoJSON.

How to use it

1. Search for a town or zoom the map, and click to place point A; click again to place point B.
2. Enter the height above the ground at A and at B, for example 1.7 m for eye level or the height of a mast.
3. Choose the refraction, and for a radio link enter the frequency in MHz to see the Fresnel zone.
4. Read the verdict in the panel and move along the chart to see the clearance at any point.
5. Drag A or B to try other places, then copy the share link or download the path.

How accurate is it?

Curvature and refraction

Over long distances the Earth curves away under the sight line: in the middle of a 50 km path the ground is about 49 m higher than on a flat Earth. The tool adds this bulge, d·(D−d)/(2kR), to the terrain, using the radius of curvature of the WGS84 ellipsoid in the direction of the path rather than one mean radius. Air bends light and radio waves slightly downwards, so they follow the curve a little; the factor k models this. k = 4/3 is the standard value for radio planning, about 7/6 suits visible light on an ordinary day, and k = 1 switches refraction off. In cold, still air over water or ice k changes a lot, and distant coasts can appear or vanish, so treat long views near the limit as uncertain.

The first Fresnel zone

A radio signal does not travel along a thin line but through an ellipsoid around it, widest in the middle. The radius of the first Fresnel zone is √(λ·d·(D−d)/D): 5.6 m in the middle of a 1 km link at 2.4 GHz and 17 m in the middle of a 10 km link. Terrain that enters the zone weakens the signal even when the line itself is clear, so planners keep at least 60 % of the zone free. Enter a frequency and the chart shows the zone, the panel gives the share that is clear at the worst point, and the lowest antenna height at B that keeps 60 %.

Terrain data and its limits

Ground heights come from Terrain Tiles, which combine national lidar models, EU-DEM, SRTM and coarser global data. Where lidar is behind the tile the height is usually within a metre or two; on SRTM it is often within 5–10 m, and narrow ridges can be smoothed out. The tool reads a finer or coarser level of the tiles to match the spacing of the 400 samples, so a 4 km path is sampled every 10 m and a 40 km path every 100 m. The data is bare ground: forest, buildings and masts are not in it, so add their height under "Trees or buildings".

Good to know

Forest and buildings block more than the hills

In flat country the terrain rarely blocks a short path, but a 20 m forest or a row of houses does. If the path crosses woodland or a town, enter a typical height under "Trees or buildings": it is added to the ground between A and B, not at the ends, and the verdict and the required mast heights change with it.

Heights are above the ground, not above sea level

Enter the height of the eye or the antenna above the ground at its own point. A mast on a hilltop gets the hill from the elevation data; entering the hill's altitude as well would count it twice and make the line look clear when it is not.

A clear line is not a guaranteed link

For radio the line of sight is the first test. Transmit power, antenna gain, cable losses, rain fade at high frequencies and interference decide whether the link actually carries data. Use the result to pick sites and mast heights, then check the link budget for the radios you plan to use.