cross

A cross is a hand-held surveying instrument used to establish perpendicular lines from a main survey line, usually during the laying out or measurement of offsets. The simplest form consists of a small frame or handle with two pairs of sighting vanes or slots arranged at right angles to one another. When held vertically and aligned with the survey line, it allows the surveyor to sight perpendicular directions without calculation or setup time.

The basic cross operates on the principle of perpendicular sightlines. Traditional designs feature either open sights (simple slots or wires) or prism arrangements that create 90-degree angles optically. The tubular cross, a common variant, uses four small telescope tubes mounted in a square pattern, each pointing in cardinal directions when held correctly. A user sights along one pair of tubes to align with the main course, then sights perpendicular directions using the other pair.

The cross is fastest over short distances, typically under 100 meters, where rough perpendicularity suffices for setting offset measurements. It requires the surveyor to work in the field with a second person holding a tape or measuring rod. Because it relies on human sighting and hand-holding, accuracy depends heavily on operator skill and steady conditions. Wind, poor light, and observer fatigue all degrade results.

The instrument has become less common with the adoption of optical theodolites and electronic total stations, which measure angles directly to much higher precision. However, crosses remain in use for small-scale fieldwork, construction layout, and situations where speed matters more than submeter accuracy. Some versions mount on tripods or range poles to improve stability.

The name reflects the cross-shaped arrangement of sightlines. In older literature it is sometimes called a surveyor's cross or offset cross. Distinct from the cross-staff, which is a longer graduated rod used for similar work but incorporating a scale for rough distance estimation.