Line-of-Sight / Viewshed

Click anywhere on the map to compute visibility from that point using real elevation data. The green shape is the ground you can see from there (with “Level view” on: the ground with nothing above eye level in the way); everything outside it is hidden by terrain. The faint circle is the edge of the analysed area. Then use the sliders — the result recomputes automatically. Every “?” in this panel explains the setting next to it.
m
Raises every wooded area mapped in OpenStreetMap (forests and woods) by this many metres, on top of whichever terrain source is selected, and treats the canopy as solid — in the viewshed and in the path profile. For an FPV or radio link that is usually the honest assumption: at 2.4/5.8 GHz a stand of trees blocks the link almost like a wall, and even at 433/868 MHz it costs serious signal.

Where measured canopy data is available — Lithuania, from a satellite survey of tree heights — the box is ignored and the real height of each patch of trees is used instead, to about 20 m of ground detail. That covers unmapped woodland too, and tells a young replanting apart from an old stand. The stats line under the result says “measured canopy” when this is what you are looking at.

The box is the fallback for everywhere else: one height for all woodland, since the outlines say where forest is and not how tall it is. Around 20 m suits mature Lithuanian pine/spruce/birch stands (old stands reach 25–30 m, fresh clearcuts and young growth much less). Outside the measured area, forest outlines are only fetched within a 30 km radius, and only where mappers have drawn them — a missing forest there is not proof of open ground.

The measurement dates from 2020, so anything felled or planted since is as it was then.
How far out to compute, from 200 m to 300 km. The slider is logarithmic, so small movements near the left end make fine changes. Elevation data is fetched at a coarser resolution for large radii, otherwise a 300 km circle would need tens of thousands of tiles.
How high your eye or antenna is above the ground at the point you clicked — about 1.7 m standing, 2.5 m on a vehicle roof, more on a mast. This matters enormously: on a rounded hilltop, raising it by a few metres can lift your sight line over the shoulder of the hill and reveal far more ground.

The slider goes to 1 km and is finer at the low end; type an exact figure in the box. Line of sight works the same in both directions, so putting an aircraft's altitude here shows everywhere on the ground that aircraft can be seen from (or can see).
How tall the thing you're trying to see is — the far antenna, a person, a vehicle, or an aircraft's height above the ground. From a ground station with 120 m here, the green area is everywhere the aircraft is in line of sight while flying at 120 m above the local ground. 0 m means the bare ground itself, which is the strictest possible test: from a rounded hilltop the ground just past the shoulder can sit a metre or two below your sight line even though anything standing on it is in plain view. Ignored while “Level view” is on. Goes to 1 km; type an exact figure in the box.
Normally your sight line tilts down with distance, so a modest rise can hide a valley well below you — that is why a 190 m hilltop may not see 160 m ground behind a 165 m swell. Tick this to ask the simpler question instead: looking dead level, is anything tall enough to get in the way? Only ground higher than your eye counts, and target height is ignored. Green then means “nothing above eye level in that direction”, not “you can see the ground there”.

Watch out over long distances. A truly level line does not follow the ground — the Earth curves away beneath it, by roughly 6 m at 10 km, 150 m at 50 km and 1.5 km at 160 km (about 15% more with the Optical refraction model). So past a few tens of kilometres a level line is high overhead and terrain simply cannot reach it: everything comes out clear. That is correct for a level line, but it is not an answer about whether you can see or reach a point — untick this for that. Unticking Earth curvature instead turns it into a plain comparison of heights.
Shades the visible area like a lake chart instead of filling it flat: red at the shallow edge, through green, to deep blue where there is most to spare. The colours never change — only the numbers on the scale beside the map.

Height below you is the ground measured down from your eye, zeroed on you rather than on sea level — a topographic chart of the view. Its scale stretches to whatever this result contains, because the point is to read the shape of the ground. With Earth curvature on, distant ground counts as deeper because the Earth really has fallen away beneath you.

Line-of-sight margin is how much the terrain in the way could rise before that spot goes dark. Its scale always starts at zero, so the colours mean the same thing every time: red really is nearly clipping something — a metre of trees or a mast in the wrong place would take it out — and blue really does have room to spare. That also means a view with nothing in the way comes out evenly blue rather than being spread across the whole rainbow, which would make a comfortable 60 m of clearance look like a landscape full of obstructions. Use it to find the fragile parts of a link.

Flying high over open ground there is usually nothing to measure against, and this mode has little to say. It earns its keep with Bare earth + buildings or the forest canopy switched on, where the things it is measuring clearance over are real.
How strongly the green (or, with a surface colouring, the rainbow) shading is painted over the map. Turn it down to read the contours or satellite imagery underneath.
Marks transmitter sites from OpenStreetMap — mobile phone masts, broadcast and communication towers, and antennas mounted on chimneys, water towers and rooftops — inside the analysed circle plus a 10% margin, so you can see what is just outside it too. Tap a mast for its operator and what it carries.

This is a where things are layer, not part of the visibility calculation: the masts are not treated as obstructions and their height is not used. It is there to warn you that flying close to one may mean interference on your video or control link.

Lithuania is bundled into this file — 2,207 sites captured from OpenStreetMap on 2026-09-12, so it draws instantly without waiting on any map-data server (the map tiles and terrain still need an internet connection). Where the circle reaches past the bundled area, or anywhere else in the world, it queries OpenStreetMap's live Overpass service instead, which can be slow or briefly unavailable when busy; the note under the checkbox says which one answered.

Coverage depends entirely on what local mappers have surveyed. Dedicated masts out in the country are usually mapped; rooftop installations in towns very often are not, so an empty map is not proof that there is nothing transmitting nearby. Outside the bundled area, not fetched beyond a 50 km radius, where there would be too many to draw usefully.
Advanced settings
This is the real limit on detail at long range, and it has nothing to do with the rays. The elevation grid is fetched at whatever zoom keeps the download manageable: at a 300 km radius that works out around 700 m per pixel on Standard, so nothing smaller than that can be resolved no matter how many rays you cast. Each step up buys one zoom level — half the pixel size — but costs four times the tiles to download and four times the memory. At small radii the standard setting is already at the data's native resolution, so raising it changes nothing except the wait. Use it when you want real detail over tens or hundreds of kilometres.

There is a memory ceiling of about 34 million elevation pixels so the tab cannot crash. Maximum bumps into it at some radii and then delivers exactly what High does; the status line says so when that happens, and the stats box always shows the zoom level actually used.
The number of directions in the 360° sweep — the scan traces the ground out along each one and the visible areas are drawn round what it finds.

The figure beside the slider is how far apart neighbouring rays end up at the outer edge. Compare it with the “m/pixel” in the stats box: once the rays are closer together than the elevation data is detailed, extra rays only re-read the same pixels and buy nothing. Rays are cheap — almost all the waiting is downloading the terrain — so raising this costs far less time than raising the elevation detail.
The Earth falls away beneath a straight sight line — about 6 m over 10 km and 94 m over 40 km with the radio setting below (7 m and 109 m for light) — so distant ground hides below the horizon even across perfectly flat land. The atmosphere bends the path slightly downward, which pushes the horizon a little further out; both effects are included. Leave this on for anything beyond a few kilometres.
How much the atmosphere bends the path back down towards the ground. Radio waves at the frequencies used for control and video links bend more than light, which is why radio planning uses the “4/3 Earth” model: the horizon drops away by 0.75 of the bare geometric amount instead of 0.87. The difference is about 15% — 147 m versus 171 m of drop at 50 km. Use Radio for link range, Optical for what a camera or your eyes can see. Only matters when curvature is on.
The elevation data includes bathymetry — under water it describes the sea floor, tens of metres down, not the surface. Leave this ticked and anything below sea level is treated as flat water at 0 m, which is what you want for looking across a sea or lake. Untick it only for genuine dry land that lies below sea level.
Path profile
A slice through the ground between the two points you clicked, with the sight line drawn across it. Green means the line gets through; red means the ground cuts it, and the figure says by how much and where. The dashed yellow mark is the tightest spot.

Heights start at 0 at the blue end — the left of the chart is your standing point, not sea level — so the far end reads as how much higher or lower it is than where you are.

The two ends come from the main panel: Observer height is the blue antenna and Target height the green one. With Level view ticked the line runs dead horizontal out of the blue end instead and the far antenna is ignored, exactly as on the map.

Earth curvature follows the main panel too. The ground is drawn from one elevation sample in four — the highest of each four, so no ridge top is stepped over — while the clearance figures are worked out from every sample.
Visible