intervisible

Two survey points are intervisible when an unobstructed line of sight exists between them. This is a fundamental requirement in surveying, geodesy, and theodolite work. If you can set up an instrument at point A and sight a target at point B without the line of sight being blocked by terrain, vegetation, or structures, those points are intervisible. The reverse is also true: someone at B looking back toward A will have equal visibility.

Intervisibility becomes critical when establishing survey networks and control points. A surveyor planning a triangulation chain must identify points that can actually see one another across the landscape, not just points that are geometrically convenient. In mountainous or forested terrain, this constraint severely limits which point combinations are usable. A hilltop may be ideal for visibility, but if dense woodland blocks the sightline to your next control point, those two stations are not intervisible and cannot form a direct measurement.

Practical visibility limits

Even when nothing physically blocks the line of sight, distance and atmospheric conditions matter. A target 50 kilometers away may be theoretically visible on a clear day but impossible to resolve through a theodolite at lower magnifications. Refraction and shimmer from ground heat can degrade sightlines further. Modern surveyors often conduct preliminary visibility checks using topographic maps and digital elevation models before fieldwork, plotting sight lines to confirm that distant points actually clear hilltops and ridgelines.

The term also appears in broader contexts: astronomers speak of intervisible celestial bodies, and telecommunications engineers must ensure relay stations are intervisible for line-of-sight radio links. In each case, the principle is identical: an unobstructed straight line must connect the two points through space.

Mutual visibility works both ways. If you cannot see from A to B because a ridge blocks your view, you also cannot see from B to A. This reciprocal nature means surveyors speak of intervisibility as a single property of a point pair, not a directional relationship. It is one reason why reconnaissance and scouting remain essential steps before committing to a survey design.