1. based on a system of fixed marks: a straight route (known as a recommended track, range or leading line), which comprises at least two structures (usually beacons or daymarks) and/or natural features, which may carry lights and/or top-marks. The structures/features are positioned so that when observed to be in line, a vessel can follow a known bearing with safety. (adapted from International Association of Lighthouse Authorities - IALA Aids to Navigation Guide, 1990)
  2. not based on a system of fixed marks: a route (known as a recommended track or preferred route) which is not based on a series of structures or features in line.