In pilotage and position fixing, a transit, in British English, or range, in American English, occurs when an observer and two fixed reference points form a straight line.  A transit of known reference points creates a position line which can be plotted on a map or nautical chart. The intersection of two position lines form a known position.
The further apart transit reference points are, the more sensitive the visual effect is to lateral movements of the observer. If the points are near to each other, larger lateral movements of the observer are needed to show up as changes in alignment. Transits can be formed from natural features or they can be deliberate navigation aids such as leading lines.
Transits usually rely on visual reference points. That can be their downfall; darkness, fog, other forms of precipitation or physical barriers can make them unusable. Leading lights are a way of providing transits at night.
- Pat Manley, Practical Navigation for the Modern Boat Owner,John Wiley & Sons,2008,p68,ISBN 978-0470516133