Line of Sight Helper
Estimate geometric horizon range and compare elevations between points for rough LOS ideas.
By Aryan Khan, Founder & Editor · Updated October 2026
For example, two 30 m towers on flat ground can see each other's tops at 39.1 km — each contributes its own 19.6 km horizon, and the sight line just grazes the curve between them.
How to use Line of Sight Helper
Two horizons, added — then terrain gets its say:
The Line of Sight Helper estimates geometric line of sight between two points: it combines both endpoints' horizon distances and compares their elevations for a rough visibility verdict. Enter the two heights and read whether the curve of the Earth alone permits the view.
- 1Set observer height.
- 2Read horizon distance.
- 3Compare elevations along the path.
What people use Line of Sight Helper for
1. Tower-to-tower and rooftop links
Before radio specifics (Radio Line-of-Sight handles those), the pure sight question: can these two tops see each other over the curve at all?
2. Viewpoint pairing
Summit-to-lighthouse, house-to-water-tower: paired elevations plus distance give the geometric verdict — terrain between them is checked on the Elevation Profile.
3. Camera and sensor placement
CCTV, wildfire lookout cameras and wildlife monitors all ask the same question this page prices: what ground is mutually visible?
4. School and scout demonstrations
Two hilltops, two torches, one calculation — the helper is the worksheet version of the horizon's geometry.
Worked examples
Two 30 m towers: 19.6 + 19.6 = 39.1 km of combined range. Put them 45 km apart and the Earth's curve alone forbids the view — no mast paint or optimism changes it; taller towers or a relay are the only votes that count.
A 500 m hill (horizon 79.8 km) watching a harbour buoy (height ~3 m, horizon 6.2 km): range ≈ 86 km. One high endpoint does nearly all the work — height is the currency, and it is not shared equally.
Combined sight ranges (geometric)
Sum of the two endpoints' horizon distances — the pattern this helper applies.
| Endpoint heights | Combined range |
|---|---|
| Eye level ↔ eye level (1.8 m) | 9.6 km |
| 10 m ↔ 10 m | 22.6 km |
| 30 m ↔ 30 m | 39.1 km |
| 100 m ↔ 30 m | 55.3 km |
| 500 m ↔ 3 m | 86.0 km |
| 1,000 m ↔ 1,000 m | 225.8 km |
Ranges assume a smooth spherical Earth and clear air. Real paths add terrain, buildings, trees — and refraction, which typically stretches these figures by about 8%.
How to interpret the result
The most common errors come from using a correct result to answer the wrong geographic question. Keep these points in mind for Line of Sight Helper:
- Treat climate and hardiness outputs as 30-year character, not this year's weather: a zone tells you what winters usually allow, and microclimates — walls, slopes, urban heat — shift a real garden half a zone either way.
- Remember that elevation models see bare terrain: no buildings, no forest canopy. A line-of-sight 'clear' verdict can still be blocked by a wood, and a horizon distance assumes open ground — treat results as the terrain baseline, then add land cover from local knowledge.
Limits & appropriate use
Terrain intelligence carries one dominant limit: the model sees bare earth at ~90 m spacing with metres of vertical error, and is blind to canopies, walls and wires. Every downstream answer inherits it — profiles show terrain truth with noisy amplitude, line-of-sight verdicts are pending trees, horizons assume open ground, and solar radiation averages assume an unshaded panel. The second limit is temporal: climate letters and hardiness zones are thirty-year character, not this year's behaviour, and seismicity is a mood, not a hazard model. Each result here is printed with its recipe precisely so these boundaries travel with the number.
The value proposition is screening-grade truth at zero cost: orientation for sites, gardens, antennas, shoots and curiosity, with the upgrade points clearly marked. When money or safety attaches to the answer — tower siting, flood insurance, structural shading, avalanche terrain — the escalation is professional: licensed survey, certified shade studies, regulatory flood determinations and geological hazard services. A good free tool makes that ladder visible instead of pretending to be the top of it.
- Construction and tower siting → licensed topographic survey.
- Flood decisions → regulatory determinations (FEMA/FIRM in the US).
- Solar finance → certified shade and production studies.
Data & methodology
The workspace adds theoretical horizon distances from observer heights; it does not inspect terrain, buildings or actual line-of-sight obstructions.
For source details, licences and model limitations, see Data Sources and Methodology.
Key terms worth knowing
- Köppen class
- The letter-code climate system from temperature/precipitation thresholds.
- DEM
- Digital elevation model: a grid of terrain heights, e.g. Copernicus GLO-90.
Editorial guidance: MapBench explains the method and limitations so the tool can be used as a transparent reference, not as a black-box answer. For legal boundaries, safety-critical routing, surveying, engineering, regulated decisions or official statistics, verify the result with the responsible authority or a qualified professional.
Category: Earth Science. Results are intended for everyday analysis and planning unless a page explicitly states otherwise.
Method, assumptions & limits
- This tool’s method
- The workspace adds theoretical horizon distances from observer heights; it does not inspect terrain, buildings or actual line-of-sight obstructions.
- Before relying on the answer
- Check the location, height units and any model options shown by this calculator. A terrain lookup and a geometric Earth model answer different questions.
- What the result cannot establish
- Local terrain, atmospheric conditions and source resolution can matter more than numerical rounding. Do not use a planning result for safety-critical work.
Coverage: Worldwide. Search, basemap and external data requests may use third-party services; browser-based does not mean every operation is offline. Read the source and licence notes, calculation methodology and privacy policy. If a result looks inconsistent, report a correction with the inputs and mode used.
Frequently asked questions
Clear answers about the calculation, accuracy and appropriate use.
Fresnel / radio LOS?
Geometric only — RF planning needs frequency and obstruction models.
Is this suitable for engineering or legal decisions?
Treat the result as planning or educational context unless the page explicitly says otherwise. Engineering, hazard and legal decisions should use authoritative specialist data.
Why can terrain data differ between sources?
Elevation and earth datasets vary in resolution, collection date and processing method. Small local features may not be represented equally.
Can I verify the data source?
Yes. MapBench documents major providers and methodology on the Data Sources and Methodology pages.
What does 'rough' mean in the verdict?
The helper judges the planet's curve and the endpoints only. A ridge, a wood or a city between the points can block a geometrically open line — profile the path for the full answer.
Where does refraction fit?
Standard refraction lengthens both horizons (the 7⁄6-radius model), adding roughly 8% to ranges on a typical day. The helper's headline numbers stay geometric so the margin is yours to see.
One endpoint at sea level?
Then its horizon is ~0 and the range is just the high end's horizon — a ship's masthead sighting a lighthouse works exactly this way, in reverse.
Heights above sea level or above ground?
What matters is height above the surface the sight line grazes — hilltop elevation plus structure height, in the terrain's own terms. A 30 m tower in a valley is not a 30 m advantage over the ridge next door.
Can I check a specific obstruction?
That is the Hidden Height and path-profile work: this helper sets the outer bound (can the endpoints see each other at all over the curve), profiles settle the middle.
Night viewing — lights change it?
A bright light can be detected slightly beyond the visual horizon by its glow and loom — the geometry of the lamp itself is unchanged, and this page prices the lamp, not the loom.
Underwater or underground?
No — line of sight here is the surface geometry of light travelling straight. Sound, radio diffraction and seismic paths each bend by their own rules.
Longest possible surface sight line?
Summit-to-summit records run past 480 km in exceptional clarity (the famous Pyrenees–Alps photographs) — combined horizons of two high peaks plus generous refraction. The arithmetic scales; the air rarely cooperates.
Related tools and resources
One horizon: Horizon Distance; radio paths: the Radio Line-of-Sight Calculator; the path's middle: the Earth Bulge Calculator.
More free tools
Sources, methodology & editorial trust
- ✓ Browser-first processing where supported
- ✓ External sources are documented
- ✓ No account required for public tools
- ✓ Review data sources
Calculations are documented with their assumptions, limitations and source context rather than presented as unexplained answers.
Reviewed and maintained by Aryan Khan, Founder & Editor of MapBench.
Part of our elevation & terrain hub
Line of Sight Helper sits in the elevation & terrain cluster, alongside 40 other tools that answer neighbouring questions with different methods or datasets.
You may also need
When this answer is only part of the job, these clusters cover the next step.
Read the guide behind this tool
How Far Can You See? The Quiet Math of Horizons — From a beach, a cliff or a cabin deck: horizon distance, observer height and terrain all matter. A practical walk through visibility math.