“Line of site” has a documented, specialized meaning in ballistics: the straight line from a projectile’s trajectory origin to its target. For ordinary visibility—or a clear path between radar or communications endpoints—the conventional term is line of sight. Which spelling is right depends on the context.
What does “line of site” mean?
In ballistics and small-arms terminology, “line of site” means a straight line from the origin of a projectile’s trajectory to its target. The Government of Canada’s TERMIUM Plus terminology record lists the phrase for that specialized use and separately identifies “line of sight” as officially approved terminology. TERMIUM Plus dates the record to May 7, 2007.
| # | Preview | Product | Price | |
|---|---|---|---|---|
| 1 |
|
Garner's Modern English Usage | $50.54 | Buy on Amazon |
| 2 |
|
Merriam-Webster's Dictionary of English Usage | $59.17 | Buy on Amazon |
| 3 |
|
The Oxford Dictionary of American Usage and Style | $17.31 | Buy on Amazon |
| 4 |
|
Fowler's Dictionary of Modern English Usage | $37.99 | Buy on Amazon |
| 5 |
|
A Dictionary of Modern American Usage | $33.99 | Buy on Amazon |
As an Amazon Associate I earn from qualifying purchases.
Dictionary.com also records “line of site” as a military term, defining it as a 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
Is “line of site” a misspelling of “line of sight”?
Not always: “line of site” is documented in the narrow ballistics sense above. But for everyday visibility, geography, radar, or communications, the usual phrase is “line of sight.” Use that spelling when you mean an unobstructed path between an observer and something viewed, or between two endpoints.
#1 Best Overall
| Form | Meaning and context |
|---|---|
| “line of site” | A specialized ballistics or small-arms term for the straight line from a trajectory’s origin to its target, as listed by TERMIUM Plus. |
| “line of sight” | An unobstructed straight path between points, used for ordinary visibility and radar geometry. |
What does “line of sight” mean in visibility and radar?
Geometric line of sight is a straight, unobstructed path between two points. In radar, MathWorks defines the radar line of sight as the unobstructed straight-line geometric path between the radar and the target. MathWorks’ radar line-of-sight explanation notes the geometric concept; Earth’s curvature, terrain, and other obstacles can limit the path.
A clear geometric path does not, by itself, establish that a radio or radar signal will be strong or usable. Signal performance involves propagation considerations beyond whether the straight path is obstructed.
Rank #2
- Used Book in Good Condition
How is line of sight assessed over terrain?
Visibility analysis in geographic information systems can determine which portions of the path between an observer and a target are visible or hidden by terrain. The heights assigned to the observer and target above the terrain surface matter, so a result can change when those offsets change. Esri’s ArcGIS 10.1 help describes this visibility-analysis concept; its page is older and is useful here for the stable principle, not current interface directions. Esri: What is visibility?
Terrain is not the only possible obstruction: buildings and vegetation can also block a visual path. Binoculars may help someone inspect a distant view, but they do not replace terrain or radio-path analysis when those are required. Rapid WiFi’s line-of-sight explainer discusses these examples.
Quick Recap
Best Value
Rank #4
Rank #3
Which spelling should you use?
- Write line of sight for everyday visibility, a clear route between two locations, or radar and terrain visibility.
- Keep line of site when referring specifically to its documented ballistics usage or quoting that terminology.
- If a phrase is ambiguous, name the field—such as ballistics or radar—so readers know which meaning you intend.
Product prices and availability are accurate as of the date/time indicated and are subject to change. Any price and availability information displayed on Amazon at the time of purchase will apply.
