Transits and Leading Lines

First identify both objects and confirm the intended line on the chart or publication. Similar buildings, seasonal foliage, background lights and an obscured lower mark can create a convincing false pair. A transit observation is valuable because it does not depend on the vessel's compass, but its accuracy still depends on correct identification and visibility.

When the objects align, the vessel is somewhere on that position line—not at one unique point. Combine the line with another independent observation such as a depth contour, cross bearing, distance, abeam feature or electronic position when a two-dimensional fix matters.

A leading line is normally followed by keeping its marks aligned. When they open, the direction and rate of opening show cross-track movement. The pilotage plan should state the correction side and a clearing limit so the helm does not improvise from a confusing night scene.