Leading line: alignment depends on where you observe

A visual alignment, or transit, relates marks on the same line: from certain positions they appear in the same horizontal direction. The front mark is nearer the observing side and the rear farther away.

When the observer moves laterally off that line, the directions separate. This visually identifies a geometric relationship, but alignment alone supplies neither distance to the marks nor a unique position.

The plan locates marks 1 front and 2 rear, together with three stationary observers. Lower panels show the horizontal relationship seen by each observer.

At A, front and rear align horizontally. They may still appear at different elevations, as is usual where the rear has suitable height.

Sharing the same vertical plane does not require the two objects to obscure each other completely in the view.

The drawing’s colours distinguish the two marks only. They are not the red/green code of a lateral mark.

For a leading alignment, the authority selects suitable features that distinguish marks from the background. Published identity and function matter: two windows or buildings that happen to line up in a view do not automatically become an established leading alignment.

The difference arises because the marks are at different distances: lateral displacement affects direction to the nearer object more. If the image is rotated or mirrored, or viewed from beyond the marks, the geometry must be reconstructed.

Memorising “front to the right” without stating viewpoint removes the reference that makes the phrase meaningful. The figure links each position with its view to retain that information.

Imagine A nearer and then farther away while remaining on the same extension. The marks remain horizontally aligned but distance changes.

Several possible positions produce the same visual relationship. Apparent vertical separation may change too, but the figure supplies no elevations, scale or distance method.

Determining a position requires additional appropriate information and assessment of its accuracy and geometry.

A geometric line extends indefinitely, but its useful segment has limits and a purpose. IHO S-4 distinguishes a navigable track from the remaining extension.

An alignment can also delimit a condition rather than be a line to follow: charts and documentation establish its function. Alignment alone resolves neither depth, obstacles nor restrictions.

Consult published purpose and extent before assigning operational meaning to the visual relationship.

English terminology distinguishes a transit, the visual relationship between aligned objects; a leading line, a published navigable lead; and a clearing line, an alignment defining a danger boundary. In IHO usage, leading lights is reserved for a lead to follow; lights in line may identify a limit.

The same visual geometry serves different functions that the legend and documentation must declare.