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A line of sight is the direct line between an observer and a target. If terrain, a building, or another object interrupts that line, the target may not be visible. The term also has precise meanings in surveying, GIS, and wireless communications, so its practical meaning depends on the field.
What does line of sight mean?
In everyday use, line of sight means a straight, unobstructed view from one point to another. Esri’s GIS Dictionary describes it as a conceptual line between an observer and a target, used to analyze visibility and identify where an obstruction blocks the view. The Office québécois de la langue française gives a closely related definition: a straight line from the observer’s eye to the object being viewed (1992 terminology entry).
In plain terms, if you can look directly from the observer to the target without an intervening feature blocking the view, they are in line of sight. The observer’s and target’s positions and heights matter, as do anything in between them.
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Any feature that lies across the direct observer-to-target path can obstruct visibility. Common examples include hills, buildings, trees, and other terrain or objects. Whether a target can be seen depends on the locations and heights involved, not just the distance between the two points.
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How does line of sight differ in technical fields?
Surveying instruments
In surveying, the line of sight is defined by the instrument’s optics: ISO 7078:2020 describes it as the line through the optical centre of the telescope’s objective lens and its cross hairs. A levelling instrument establishes horizontal lines of sight. This is an instrument-defined direction, rather than simply an observer’s unaided view.
GIS and terrain visibility
GIS line-of-sight analysis evaluates visibility between observer and target points over a surface and can identify the first obstruction. Results depend on the surface data, point locations and heights, and analysis settings. ArcGIS Pro documentation notes that calculations can optionally account for Earth curvature; its refraction correction compensates for atmospheric bending affected by air pressure, density, humidity, temperature, and elevation (Esri’s Line Of Sight documentation).
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Radio and microwave links
For a radio or microwave link, line of sight refers to a direct propagation path between sites. Seeing one site from another is only an initial check, not proof that the path is suitable for a microwave signal. The U.S. Department of Transportation’s Telecommunications Handbook for Transportation Professionals identifies terrain and features such as buildings, trees, hills, and elevated roads as possible obstacles, and cautions that visual confirmation alone does not guarantee an appropriate microwave path.
A specialized IT meaning
The Federal Geographic Data Committee lexicon also records a specialized use in IT investment: a direct or indirect cause-and-effect relationship between an investment and the processes, customers, and mission outcomes it supports (FGDC lexicon). This is not the ordinary visual meaning.
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Does seeing a site mean a wireless signal will reach it?
No. A visible path is useful as a first check, but it does not establish that a microwave link will work. The DOT handbook warns that being able to see one site from another does not guarantee the visible path is appropriate for a microwave signal. A communications assessment must address radio-path suitability, not just whether the sites look unobstructed.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Which meaning should you use?
- Everyday viewing: the direct visual path between a person and an object.
- Surveying: the direction defined by an instrument’s telescope optics and cross hairs.
- GIS: a modeled visibility path between observer and target points across terrain or a surface.
- Wireless communications: a direct propagation path whose suitability cannot be confirmed by sight alone.
- IT investment: a causal link between an investment and the processes or outcomes it supports.
When the distinction matters, name the field and identify the observer and target, their heights, and any intervening features. For GIS, also establish whether Earth curvature and atmospheric refraction are relevant to the analysis.
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