Method and limitations
How Sightelo works
Sightelo estimates a geometric line of sight between an observer and a target. It is a topographic model, not a promise that the target will be visible in real-world conditions.
A custom-built application
Sightelo is a custom-built web application. Its Python calculation engine uses Copernicus elevation data; MapLibre displays the map and Chart.js displays the profile.
1. Select two points
Point A is the observer. Point B can be a summit, monument, building or geographic point. Addresses and place names are converted to coordinates only to position these points.
2. Build the terrain profile
Elevation samples are obtained from Copernicus DEM along the geodesic between A and B. Known summit elevations supplied by the place database are preferred because a 90-metre terrain pixel can smooth a sharp summit.
3. Apply Earth curvature
The apparent terrain profile includes Earth’s curvature. At long distances, curvature can hide a low target even when there is no intervening mountain.
4. Approximate atmospheric refraction
Standard refraction bends light slightly towards the surface and usually extends the geometric horizon. Sightelo uses a conventional coefficient of 0.13 when this recommended option is enabled. Actual refraction varies with temperature and weather.
5. The minimum sight line
The highest mountain is not necessarily the obstacle that matters: a lower ridge nearby can appear higher from the observer. Sightelo compares the apparent angle of each sampled terrain point, after correcting for curvature and the selected refraction. The greatest angle defines the terrain horizon.
The minimum sight line starts at the observer's eyes and just clears that horizon. Its height at the target gives the minimum visible elevation. A target above it is geometrically visible; a target below it is hidden. The horizon-forming point is marked on the profile even when the target is visible. If curvature alone settles the result, no intermediate terrain profile is sampled.
6. Report a confidence level
A result is marked uncertain when its clearance is close to the selected elevation uncertainty. Buildings, trees, haze, clouds and local atmospheric effects are not included.
A small margin above or below the horizon can change sign with elevation errors or different refraction. A close local passage or disagreement between the centre line and the corridor can also make a result uncertain. The margin is a geometric estimate, not a probability of seeing the target or a guarantee of optical detail.
Reading the two profile views
Visibility view
The blue 0 m reference follows the modelled curvature. Terrain is drawn at its real elevation plus the curvature correction; sight lines stay straight. Real elevation is the difference between the terrain and the blue reference at the same distance.
Topography view
The blue 0 m reference is horizontal and terrain is drawn at its real elevation. The same curvature correction is subtracted from the sight lines, so they become curved. Both views describe the same calculation and give the same visibility result.
Distances follow the WGS84 geodesic and each point is placed at its actual distance. The vertical scale is amplified to make relief readable: slopes on screen are not real viewing angles. Hover over a point to read its real elevation, the curvature correction at that point and the gap between the target sight line and the minimum sight line.
Elevation references and curvature
WGS84 is the reference ellipsoid used for positions and distances. Copernicus elevations are orthometric heights relative to the EGM2008 geoid, a reference approximating mean sea level. The blue curve is a modelled 0 m reference, not an exact drawing of the ellipsoid or geoid.
Curvature uses a mean Earth radius of about 6,371 km and a regional-distance approximation. Refraction is represented by an effective radius R / (1 − k): k = 0 disables it, while k = 0.13 is the standard option. This does not model changing atmospheric layers along the route.
Terrain resolution and excluded areas
The calculation starts with DEM 90 and can refine borderline results with DEM 30 when available. A narrow corridor samples the centre and both sides of the route, retaining the highest intermediate terrain. The display can use fewer points than the calculation while retaining the horizon-forming point.
By default, the first 200 m around the observer are excluded as obstacles because a coarse terrain pixel cannot describe the immediate surroundings reliably. Near a summit, an adjustable final area is treated as part of the target rather than a separate obstacle. These exclusions change the horizon being tested; check the actual radii in the calculation assumptions.
Privacy
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