UpToWhereViewshed analysis
What a viewshed is, what actually decides whether one point can see another, and how to compute one without a GIS install.
Free, no account needed. One point, one question.
A viewshed is the set of ground that a given observer can see. You fix a point and a height, and the analysis returns every place within range that has an unobstructed sightline back to it, and every place that does not.
What decides whether a point is visible
Four things, and only one of them is the terrain in between. The other three are why two people standing in the same field get different answers.
The ground between
Any ridge, hill or bank that rises above the straight line between the two points blocks it. This is the part everyone expects, and it is rarely the part that decides a marginal case.
The observer height
Height above ground, not above sea level. Raising the observer lifts the whole sightline, and it lifts it most at the far end. This is why a 30 m mast sees so much further than a 2 m person on the same spot.
The target height
You are usually not asking whether you can see the soil. A turbine tip, a rooftop, an antenna: the thing you are looking for has its own height, and it can be visible when the ground under it is not.
The curvature of the Earth, and the atmosphere
Over any real distance the planet gets in the way, and the air bends the sightline back down over it. Both are geometry, and both are computed here rather than ignored.
How far you can see from a given height
With nothing in the way at all, the limit is the horizon, and the horizon is set by your height. The geometric answer is the square root of twice the Earth radius times the height. The two right-hand columns are the same formula with the atmosphere put back in: light bends slightly downward as it travels, so the true horizon sits further away than the geometry alone says, and radio bends more than light does.
| Height above ground | Geometric | Optical (k = 1.15) | Radio (k = 4/3) |
|---|---|---|---|
| 2 m | 5.0 km | 5.4 km | 5.8 km |
| 10 m | 11.3 km | 12.1 km | 13.0 km |
| 30 m | 19.6 km | 21.0 km | 22.6 km |
| 50 m | 25.2 km | 27.1 km | 29.1 km |
| 100 m | 35.7 km | 38.3 km | 41.2 km |
| 300 m | 61.8 km | 66.3 km | 71.4 km |
Distance to the horizon for a target at ground level. If the thing you are looking at is also raised, add its own horizon distance: two 30 m masts can see each other at roughly twice the single-mast figure, terrain permitting.
The formula
And how much Earth is in the way
Between two points a distance apart, the planet bulges up in the middle. This is what a long link has to be raised over, and it grows with the square of the distance.
| Path length | Optical bulge | Radio bulge (k = 4/3) |
|---|---|---|
| 5 km | 0.4 m | 0.4 m |
| 10 km | 1.7 m | 1.5 m |
| 25 km | 10.7 m | 9.2 m |
| 50 km | 42.7 m | 36.8 m |
| 100 km | 170.7 m | 147.2 m |
Mid-path bulge, in metres. At 50 km, the difference between the optical and the radio constant is about 6 m of mast, which is the difference between a routine pole and a structure that needs planning.
Bare earth or surface: the choice that changes the answer
Every viewshed is computed on an elevation model, and elevation models come in two kinds. A digital terrain model (DTM) is the bare ground with vegetation and buildings stripped off. A digital surface model (DSM) is the top of whatever is there, so a forest canopy and a rooftop are part of the terrain.
Neither is right in general, and picking the wrong one is the most common way a viewshed misleads.
Use bare earth for planning and landscape work
A zone of theoretical visibility is meant to be the worst case: trees are felled, buildings come down, and a screening belt you counted on may not be there in twenty years. Bare earth is what makes a visual impact assessment defensible.
Use a surface model for radio and sensor work
A microwave link does not care that a stand of pines is technically not terrain. If it is in the path, it is in the path. A surface model puts the canopy where the beam meets it.
Know which one you are on
The global 30 m Copernicus GLO-30 dataset is a surface model, so trees and rooftops are partly inside the heights already. Read that sentence again if you are producing a ZTV.
Resolution is not accuracy
A 1 m grid tells you where the ground is more finely; it does not tell you whether the model is right about it. Both numbers matter and they are different numbers.
Every result computed here states the model it used, its licence and its attribution, on the result and inside every export. Bare-earth national terrain is available for eight countries at 0.5 m to 10 m, and everywhere else falls back to the global 30 m surface model. What each dataset is, and what it is not, is set out on the methodology page.
Running one
- Put the point where the observer actually isA summit entered from memory usually lands on the slope, and a mast 100 m below its own crest is blocked by the hill it stands on. Drop the pin on the map, not from a remembered coordinate.
- Set the height above groundNot above sea level. The eye height of a walker, the hub or tip height of a turbine, the mounting height of an antenna. This one number moves the answer more than any other input.
- Read the result, then check the edge casesThe interesting parts of a viewshed are its boundaries. Anywhere the answer flips over a few metres of height is a place worth a second look and, eventually, a site visit.
When one observer is not the question
A single viewshed answers "what can this see". Planning and network questions are usually about several structures at once, and the useful answer is not the union of their viewsheds but the count: from any given point, how many of them are in view.
That is a cumulative viewshed, and in landscape and planning work it is called a zone of theoretical visibility, or ZTV. One turbine visible from a village is a different finding from eleven, and only the count distinguishes them.
Getting it into GIS, or into Google Earth
A viewshed is only a deliverable once it leaves the tool it was computed in. Three formats cover almost every downstream use, and results here export to all of them.
GeoTIFF
A georeferenced raster that QGIS, ArcGIS and GRASS open directly, with the basis of the calculation written into the file tags.
KML and KMZ
Opens in Google Earth. This is usually what a client or a consultee can actually look at without installing anything.
PNG plus world file
A plain image with a six-number sidecar that places it on the map, for anyone whose workflow expects an overlay rather than data.
Every export carries a provenance line naming the dataset, the engine version and the date, plus the sentence that matters most: a modelled result is not a survey.
Common questions
What is a viewshed, in one sentence?
The set of ground visible from a given point at a given height, and by implication the set that is not.
What is the difference between a viewshed and a ZTV?
A viewshed is one observer. A ZTV, or cumulative viewshed, combines several structures and reports how many of them are visible from each point, which is the question a planning consultee is actually asking.
Does it model buildings and trees?
It depends on the elevation model. The global 30 m dataset is a surface model, so canopy and rooftops are partly included in the heights already, but they are not modelled as objects and you should not rely on either reading. Bare-earth national datasets have them stripped out. Whichever was used is stated on the result.
How accurate is it?
That depends on the terrain data underneath, not on the geometry. The geometry is standard and testable, and the results here are checked against published field surveys and photographed long-distance sightlines. Those comparisons, including where they disagree, are published on the validation page.
Can I use a viewshed in a planning or environmental report?
Modelled visibility is a normal part of landscape and visual assessment, and what makes one usable is that its data, method and limits are stated. Every export here carries that statement. It is not a substitute for a licensed survey and it is not a certified measurement, and no result should be relied on for a safety-critical purpose.
Do I need GIS software?
No. The calculation runs in the browser interface and the result can be read there. GIS is only needed if you want to combine it with your own layers, which is what the GeoTIFF and KML exports are for.
How far can you actually see?
From two metres above flat ground, about 5 km. From a 30 m mast, about 23 km. From a 300 m summit, about 71 km, and considerably further towards another summit. The table above is the arithmetic.