Transit

A transit (or range in US terminology) occurs when two charted objects are observed to be in line simultaneously, providing a precise and instantaneous position line independent of compass error. The Admiralty Manual of Navigation describes transits as among the most reliable of coastal navigation techniques because they require no instrument other than the eye and produce a bearing line on the chart drawn through the two charted marks. When two simultaneous transits cross, they define a fix; a single transit combined with a depth sounding can also confirm position in waters with a distinctive bottom profile. In pilotage waters, leading lines and clearing lines are forms of transit designed into the port approach and marked on the chart. The OOW or pilot uses these transits to verify that the vessel is on the correct track or, when a charted hazard opens or closes on a transit, that the vessel is safely clear. ECDIS systems can display leading lines, but direct visual observation of transits remains the most immediate and reliable check, particularly in the absence of reliable GPS. Transits should be identified on the passage plan and monitored throughout the approach as a continuous cross-check on the electronic position.