Viewshed Analysis and Line of Sight Calculator

What a viewshed analysis is for

A viewshed analysis shows which parts of the terrain can be seen directly from an observer at a given height. For every cell, the tool checks whether the straight line from the observer to that cell hits the ground on the way. If it does not, the cell counts as visible.

The same question comes up in many kinds of work. Telecom and fibre teams use it to screen which settlements a base station or microwave tower can reach. In early solar and wind studies, how visible a plant is from nearby villages and roads is part of the visual impact assessment. Security camera and watchtower layouts use it to find blind spots, and emergency planners use it to choose lookout points, siren locations or radio repeater sites.

How to use it

1. In Viewshed mode, click the observer location on the map or type its coordinates. Enter the tower or mast height in the list. You can add up to three observers; in the combined result, ground seen from two or more observers gets its own colour.
2. Set the target height (how far above the ground the point to be seen is), the radius and the refraction coefficient, then press Compute Viewshed. The summary gives the visible area in km² and as a share of the analysed area.
3. In Line of Sight (A-B) mode, pick two points, enter the antenna heights and, optionally, the frequency. The chart shows the terrain profile, the sight line and the first Fresnel zone; if something is in the way, the tool reports where it is and how much the antennas need to be raised.
4. Download the viewshed as ZIP (PNG, PGW, KML), KMZ or GeoJSON, and the profile as CSV.

Line of sight and the Fresnel zone

For a radio link, a clear straight line between the two antennas is not enough. Much of the energy travels inside the ellipsoid around that line known as the first Fresnel zone. Its radius at any point is r1 = √(λ · d1 · d2 / D), where λ is the wavelength, d1 and d2 are the distances from the point to each end and D is the total path length. At 5 GHz the wavelength is about 0.06 m, so at the midpoint of a 10 km link r1 comes out at about 12.2 m. In practice at least 60% of this zone should be free of obstacles. The tool computes the clearance at every profile point, marks the critical point and reports the extra height needed to achieve line of sight and 60% clearance by raising A only, B only or both together. When a frequency is entered, the free-space path loss is also shown.

Earth curvature and refraction

Over long distances the curvature of the earth drops the target below the sight line. The drop at distance d is d² · (1 - k) / 2R, where R is the mean earth radius and k is the atmospheric refraction coefficient. The atmosphere bends rays slightly towards the ground and reduces the curvature effect. A common default for visual line of sight is k = 0.13, which gives a drop of about 6.8 m at 10 km. Radio links in a standard atmosphere are usually planned with an effective earth radius factor K = 4/3, equivalent to k = 0.25.

Data and method

Elevations are read from the open AWS Terrain Tiles dataset (Mapzen/Tilezen). For Türkiye and most of the world, the source data largely comes from global models such as SRTM with a resolution of roughly 30 m. For the viewshed, the area around the observer is divided into a latitude-longitude grid of at most about 360,000 cells, so the spacing is about 17 m at a 5 km radius, 34 m at 10 km and 67 m at 20 km. Visibility is computed with the R3 method: a separate ray is traced from the observer to every target cell, curvature-corrected elevations are read along it and the angle to the target is compared with the steepest angle on the way. Additional observers snap to the nearest cell centre. The line of sight profile is sampled about every 20 m from the most detailed tile level.

The profile chart runs on Chart.js 4.4.1 (MIT licence) and ZIP and KMZ files are built with JSZip 3.10.1 (used under its MIT licence option); icons come from Lucide (ISC licence). Basemaps are provided by Esri and OpenStreetMap contributors. Open-source libraries used and their licences: Data Sources.

Accuracy limits

• Bare earth: the model does not include buildings, trees, masts or recent cut and fill as separate features. In built-up areas and forests the real visibility can be noticeably smaller than the map suggests.
• Resolution: the source model's true resolution is about 30 m. Narrow ridges, cuttings and valley floors are smoothed, and the effect grows as a larger radius widens the cell spacing.
• Observer position: a small position error on a summit can put the observer below or above the model surface; at critical sites, try a few heights.
• Preliminary screening: the results are for early assessment. A firm radio link design needs a site survey, measurements where necessary and licensed planning software that accounts for vegetation and structures.

For professional projects

Rodosto Teknoloji works on GIS projects for telecom, energy and site selection; you can find an overview on the company website. For questions about a specific project, use the contact page.