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“Line of site” is a valid but specialized ballistics term: it means the straight line from a projectile’s trajectory origin to its target. For everyday visibility, terrain, radar, or communications, the usual phrase is “line of sight”—an unobstructed path between two points.
What does “line of site” mean?
In ballistics and small-arms terminology, “line of site” refers to the straight line from the origin of a projectile’s trajectory to its target. The Government of Canada’s TERMIUM Plus terminology record, dated May 7, 2007, lists the phrase as correct in that context and defines it as “In ballistics, straight line from the origin of the trajectory to the target.” TERMIUM Plus
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Dictionary.com also records a military meaning: the straight line from an artillery gun’s muzzle to its target. Its entry dates the phrase’s first recorded use to 1905–10; that is the dictionary’s dating, not proof of the exact first use. Dictionary.com
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Not always. “Line of site” has a documented specialized use in ballistics, but it is not the conventional spelling for general visibility. TERMIUM Plus separately identifies “line of sight” as officially approved terminology. If you mean that a person, signal, or radar can reach something along an unobstructed path, write “line of sight.”
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| Phrase | Use |
|---|---|
| “Line of site” | Specialized ballistics or small-arms usage: a line from a trajectory’s origin to its target. |
| “Line of sight” | General visibility and geometric path between endpoints, including radar contexts. |
What does “line of sight” mean in technology and geography?
In radar geometry, line of sight is the unobstructed straight path between the radar and its target. MathWorks describes it as “the unobstructed straight-line, geometric path between the radar and the target.” MathWorks
The same geometric idea applies to visual visibility and terrain analysis. Earth’s curvature, hills, buildings, and vegetation can block the path. GIS visibility analysis can identify which portions of a path from an observer to a target are visible or hidden; the observer’s and target’s heights above the terrain affect the result. Esri’s ArcGIS 10.1 help describes this visibility-analysis concept, though it is not current interface guidance. Esri
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Does line of sight guarantee a working radio or radar signal?
No. Geometric line of sight describes whether a straight path is obstructed; it does not by itself establish signal strength or guarantee a usable radio link. Terrain and other obstacles can affect visual or radio paths, and Earth curvature can limit the geometric horizon. A communications assessment may require radio-path or engineering analysis beyond checking whether endpoints appear to have a clear view. MathWorks Rapid WiFi
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Which phrase should you use?
- Use “line of site” when referring specifically to the documented ballistics term or quoting a source that uses it.
- Use “line of sight” for ordinary visibility, terrain visibility, radar geometry, or a clear path between communications endpoints.
- If the context is unclear, write the intended meaning explicitly—for example, “the unobstructed path between the antenna and receiver.”
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