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Repair Windows errors before they cause bigger problemsFix Now →Scan for outdated or missing drivers - takes under a minuteDriver Scan →Clear out junk files and repair common Windows errorsFree Scan →A line of sight is the direct line between an observer and a target. If terrain or another object blocks that line, the target is not visible from that position. The same phrase has more specific meanings in surveying, GIS and wireless communications, so context matters.
What does line of sight mean?
In everyday language and mapping, a line of sight is a straight, conceptual path from the observer to the thing being viewed. Esri’s GIS Dictionary uses the term for visibility analysis: the line helps determine whether an obstruction lies between the observer and target. 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).
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In practical terms, a line of sight answers a simple question: is there an unobstructed direct view from here to there? The answer depends on where the observer and target are, their heights, and what lies between them.
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What can block a line of sight?
Any intervening feature that interrupts the direct path can obstruct visibility. Depending on the setting, that may be a hill, building, tree, or other object. A target may be visible from one position but hidden from another, or become visible when the observer’s or target’s position changes.
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For a map-based analysis, the observer and target are represented by locations and heights over a surface. The result is only as reliable as the underlying terrain or surface data and the assumptions and settings used. In ArcGIS Pro, for example, line-of-sight analysis can optionally account for Earth curvature; its refraction correction models atmospheric bending affected by air pressure, density, humidity, temperature and elevation. See Esri’s explanation of how line-of-sight analysis works.
How the meaning changes by field
Surveying
In surveying instruments, line of sight has an optical meaning. ISO 7078:2020 defines a telescope’s line of sight through the optical centre of its objective lens and the cross hairs. A levelling instrument establishes horizontal lines of sight. This is about the instrument’s viewing and measuring axis, not simply whether a person can see a distant object.
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GIS and terrain analysis
A GIS line-of-sight calculation assesses visibility between observer and target points across a modeled surface. It can indicate whether the path is clear and where the first obstruction occurs. Because the calculation depends on the surface data and selected options, its result describes that model and those settings; it is not an unconditional guarantee of what will be visible on the ground.
Radio and microwave links
For wireless communications, line of sight refers to a direct propagation path between sites. Seeing the other site is a useful initial check, but it does not prove that a microwave link will work. The U.S. Department of Transportation’s Telecommunications Handbook for Transportation Professionals warns that visual confirmation alone does not establish whether a visible path is appropriate for a microwave signal. Its planning discussion identifies terrain and structures—including buildings, trees, hills and elevated roads—as potential obstacles.
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A specialized IT meaning
The Federal Geographic Data Committee’s lexicon also records a distinct IT-investment use: a direct or indirect cause-and-effect relationship between an investment and the processes, customers and mission outcomes it supports. This is not about physical visibility; it describes traceability of impact.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Which meaning should you use?
Identify the field before interpreting the phrase. For ordinary viewing, it means an unobstructed direct view; for surveying, it concerns an instrument’s optical axis; for GIS, it describes a modeled visibility test; and for wireless planning, it concerns a propagation path that needs technical assessment beyond visual inspection. In IT investment discussions, it can refer to a relationship between an investment and its outcomes.
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