Place two points on the map and instantly see whether they have a clear line of sight to each other, accounting for the terrain between them, Earth's curvature, and atmospheric refraction. Open in full screen ↗
What This Tool Calculates
Two points can look perfectly visible to each other on a flat map and still be unable to see one another in reality — a ridge, hill or rise between them can block the view even when both spots themselves sit in the open. This tool draws a straight three-dimensional line between two points you place, samples the ground elevation along that line, and checks whether anything — terrain, or simply the curve of the Earth itself over a long enough distance — rises above that straight line anywhere between them. If nothing does, the two points have a clear line of sight.
Why Earth's Curvature and Refraction Matter
Over short distances, the ground's curve is negligible and terrain obstructions dominate the result entirely. But the Earth's surface genuinely does curve away beneath a perfectly straight line, and past roughly a kilometer or two that drop becomes large enough to matter — at 10 km, the surface has fallen away from a straight line by several meters, purely from geometry, with no hill involved at all. Light doesn't travel in a perfectly straight line through the atmosphere either: air gets thinner with altitude, which bends light very slightly downward, following the Earth's curve and letting you see a little further than pure geometry alone would predict. This tool corrects for both effects together at every sampled point along the path:
- Curvature drop: how far the Earth's surface falls away beneath a straight line between two points — zero at each end, largest at the midpoint, and growing with the square of the distance.
- Refraction coefficient (k): a correction for how the atmosphere bends light downward. This tool uses k = 7/6, the standard value for everyday optical and visual sighting.
- Clearance: the vertical gap between the straight sight line and the terrain, plus the curvature drop, at each sampled point. A negative clearance means something blocks the view at that point.
Real-World Use Cases
📷
Wildlife Camera & Observation Post Placement
Position a trail camera, hunting stand or birdwatching hide with a confirmed clear sightline to a trail, feeding area or nest.
🏡
Property & Scenic View Assessment
Check whether a potential building site or viewpoint genuinely has a clear view of a landmark, coastline or skyline before buying or building.
🔥
Wildfire Lookout & Security Camera Planning
Check the visible coverage between a proposed lookout tower or security camera position and the area it needs to monitor.
🚁
Drone Flight Path Planning
Verify that a planned drone route stays within visual line of sight of the pilot, often a legal requirement for recreational and commercial flights.
📶
Microwave & Telecom Backhaul Links
Confirm that two proposed tower sites for a point-to-point microwave link have a clear path before survey crews and equipment are sent out.
📡
Radio & Antenna Siting
Check whether a proposed antenna or repeater site has a clear path to another tower before committing to a location, factoring in the terrain between them.
Common Questions
What is atmospheric refraction, and why include it?
Light doesn't travel in a perfectly straight line through the atmosphere — it bends very slightly downward because air gets less dense with altitude. That bending effectively lets you see a little further than pure geometry alone would allow, which is why every serious line-of-sight calculation includes a refraction correction.
Why does the tool say the line is blocked when I can see both points fine on the satellite image?
The satellite basemap only shows what's directly at each point, not the ground profile between them. A ridge, hill or rise partway along the path can block the line even though both endpoints themselves look clear from above.
How accurate is the elevation data?
Elevations come from ASTER GDEM v3, a satellite-derived global surface model with 30-meter horizontal resolution. It's detailed enough for hills, ridges and general terrain shape, but it can miss narrow features smaller than about 30 meters across and has a typical vertical accuracy of a few meters, so treat results very close to the clear/blocked boundary as inconclusive.
Does this tool work on mobile devices?
Yes — tap the map to place each point the same way you'd click on desktop, and the map and results panel stack vertically on narrow screens.
Is my location data stored anywhere?
No. Points, heights and results all stay in your browser. The only outbound request is a single elevation lookup sent to the elevation data provider for the two points you place, with no accounts, tracking, or saved history involved.
Can I use this for radio or wireless signal planning?
It's a solid starting point for checking basic terrain clearance between two sites, but real RF link planning also needs Fresnel zone clearance — extra space around the direct path, not just the path itself — a radio-appropriate refraction coefficient, and often a vegetation-aware elevation model, none of which this tool calculates.
Why does this tool use a refraction coefficient of 7/6 instead of 4/3?
k = 7/6 is the standard value for everyday optical and visual sighting under typical atmospheric conditions. Radio and microwave link planning conventionally uses a higher value, k = 4/3, because radio waves bend slightly more than visible light. If you're planning an RF link rather than checking a visual view, treat this tool's result as a starting estimate rather than a final clearance figure.
What height should I enter for the observer and target?
Enter how far each point sits above the ground at that location — roughly 1.5–1.8 m for a standing person, or the actual height of a mast, tower, rooftop or platform. Both values default to 1.7 m, a typical eye height.
What does "line of sight" mean in this tool?
It means a straight, unobstructed 3D path between two points — factoring in both the terrain between them and the fact that the Earth's surface curves away over distance. If nothing rises above that straight line anywhere along the path, the two points can see each other.
How does the calculator account for Earth's curvature?
For lines longer than a kilometer or so, the ground itself curves away beneath a perfectly straight line. The tool calculates how much the surface drops at every sampled point along the path and adds that drop to the terrain elevation before comparing it against the sight line.
Step by Step
1Step 1
Open the tool
The map opens ready to place points, with "Placing Observer (A)" already selected — the hint above the map tells you what to click next.
2Step 2
Click the map to place Point A
This is the observer's position. The tool automatically switches to placing Point B once A is set, so you can place both points with two clicks.
3Step 3
Click again to place Point B
This is the target's position. A dashed grey line appears between the two points as soon as both are placed.
4Step 4
Fine-tune either point
Drag either marker to adjust its exact position, or use Swap to reverse A and B, or Clear to start over.
5Step 5
Set the height above ground
Enter how far each point sits above the ground — roughly 1.5–1.8 m for a standing person, or the actual height of a mast, tower, rooftop or platform.
6Step 6
Click "Check Line of Sight"
The tool fetches a 100-point elevation profile between the two points in a single request and calculates visibility, including curvature and refraction.
7Step 7
Read the result
A green "Clear" or red "Blocked" banner shows the distance and clearance at the tightest point, with the elevation profile chart below it showing exactly where.
8Step 8
Export or adjust
Download the result as a GeoJSON file for use in GIS software, change the heights to instantly recheck without a new lookup, or use "Use my location" to set a point to where you are.
A Short History of Line-of-Sight Analysis
Checking whether two points can see each other is one of the oldest problems in surveying and military planning. Armies positioned signal beacons and semaphore relay stations on hilltops chosen specifically because each one could see the next, and 19th-century optical telegraph networks across Europe were engineered almost entirely around intervisibility between towers. The same problem resurfaced in the 20th century for line-of-sight microwave relay networks, which needed a clear, obstruction-free path between towers spaced many kilometers apart to carry telephone and later data traffic before satellites and fiber took over. When raster GIS software matured in the 1970s and 80s, visibility analysis was formalized into the "viewshed" — the set of all locations visible from a given point — a technique still used today for siting cell towers, wind turbines, wildfire lookouts and radio repeaters, and for drone flight planning, where staying within the pilot's direct visual line of sight is often a legal requirement.
Where the Elevation Data Comes From
Ground elevations come from ASTER GDEM v3, a satellite-derived global surface model jointly produced by NASA and Japan's METI, covering nearly the entire planet at 30-meter horizontal resolution. Because it's a surface model rather than a bare-earth model, it captures the general shape of tree cover and large structures to some degree, not just open ground — but at 30-meter resolution it can't resolve individual buildings, towers or narrow ridgelines, and its typical vertical accuracy is on the order of a few meters. Treat a result that's very close to the clear/blocked boundary as inconclusive rather than final, and remember that trees, buildings, and other obstructions built after the source imagery was captured aren't guaranteed to be reflected in it.
Watch: Line of Sight Calculator in Under 2 Minutes
Prefer to watch instead of read? This short walkthrough covers placing two points and checking whether the terrain between them blocks the view.
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Map tiles © OpenStreetMap contributors and Protomaps (hosted by KONAKTIVE), CARTO, Esri (satellite & terrain). Elevation data from OpenTopoData (ASTER GDEM v3). All line-of-sight geometry is calculated locally in your browser — no location data is stored on any server.
