Viewshed Analysis: Understanding Line of Sight
Line of sight checks visibility between one pair of points — standing here, can I see there? — while a viewshed applies that same question to every cell in an area at once, producing a map of what is visible. Both look like a simple geometry problem, but the arithmetic is the easy part; almost all the error comes from the inputs. This guide covers the four decisions that actually determine your result.
Two related questions
Line of sight asks about one pair of points: standing here, can I see there? The computation walks the terrain profile between the two and checks whether anything rises above the straight line joining them.
A viewshed asks the same question for every cell in an area at once, and returns a map: visible or not visible from the observer. It is line of sight repeated in every direction.
STREAM computes viewsheds and can output the result as a raster or as vector polygons, so the visible area can be measured, exported and overlaid on a drawing.
Decision 1: DSM or bare-earth terrain
This is the single biggest factor, and the one most often left on the default.
A viewshed on bare-earth terrain sees through buildings and trees, because they are not in the surface. It answers "does the landform block the view" — the right question for a landscape or planning study where the vegetation may be cleared.
A viewshed on a surface model including buildings and canopy answers "is the view blocked today". The visible area will be dramatically smaller, and for anything in a built-up or forested area the bare-earth result is simply optimistic.
Neither is wrong. But a report that does not say which surface was used cannot be checked.
Decision 2: observer and target heights
The observer is almost never on the ground. An antenna sits on a mast, a camera on a pole, an eye 1.6 m above the surface. Adding the real offset changes the result far more than people expect, because visibility is decided by grazing angles over intervening ridges.
Target height matters just as much and is skipped more often. Asking "is the ground visible" and asking "is a 30 m turbine visible" give very different maps from the same terrain.
Decision 3: radius
Every viewshed needs a limit. Without one the computation runs to the edge of your data, which produces long thin visible slivers at great distance that are technically correct and practically meaningless.
Set the radius from the physics of what you are studying: radio range, the distance at which an object of that size is actually discernible, or the study boundary you have to report on.
Decision 4: how far is far
Over short distances the earth is flat enough to ignore. Over tens of kilometres it is not — curvature drops the far terrain away from the sight line, and atmospheric refraction bends the ray slightly back down. The two partly cancel, which is why radio work uses an effective earth radius rather than the true one.
If your radius is a few kilometres, this does not matter. If you are assessing visibility across a valley system or planning a long radio link, it does, and a plain geometric viewshed will be pessimistic about long shots.
What it is used for
Telecommunications: siting masts and checking link paths before anyone climbs anything.
Visual impact assessment: which settlements will see a wind farm, a quarry face or a building, and how much of it.
Security and monitoring: camera placement, and finding the dead ground a fence line cannot see.
Mining and quarrying: designing screening bunds and checking that a working face is hidden from a road or a village.
Forestry and emergency management: lookout tower placement and coverage gaps.
Where it stops being reliable
A viewshed is geometry, not optics. It reports whether a straight line is unobstructed — not whether a human eye would notice the object at that distance, in that light, against that background.
It also inherits every flaw in the surface. A DEM that smoothed away a small ridge will report a view that a site visit disproves in seconds. For anything consequential, treat the viewshed as the map that tells you where to go and check, not as the conclusion.
Frequently asked questions
What is the difference between line of sight and a viewshed?
Line of sight checks visibility between a single pair of points — standing here, can I see there? — by checking whether anything rises above the line connecting them. A viewshed applies that same question across an entire area to produce a visibility map, which STREAM can output as a raster or as vector polygons.
Should a viewshed use bare-earth terrain or a surface model (DSM)?
It depends on the question being asked: bare-earth terrain ignores buildings and trees and answers whether landforms alone block a view, while a surface model that includes structures gives a current-day visibility picture. Neither choice is wrong, but a report that doesn't state which surface was used cannot be checked.
Why do observer and target height matter for a viewshed result?
Results are highly sensitive to grazing angles over intervening ridges, so changing the observer height (e.g. an eye 1.6 m above the surface) or the target height (e.g. ground level versus a 30 m turbine) can significantly change what is visible.
Where does a viewshed stop being a reliable indicator of what someone would actually notice?
A viewshed is geometry, not optics — it only reports whether a straight line is unobstructed. It does not account for whether a human eye would actually notice the object at that distance, in that light, or against that background.
Sources
Run viewshed analysis in STREAM
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