A radar technique using lines parallel to the course line to monitor closest approach to a fixed radar target.
In practice
A single index line set to a coastal headland, lighthouse, or buoy gives a continuous, automatic position check requiring no plotting. Multiple index lines covering successive waypoints can be pre-set before the passage. Modern ECDIS systems can replicate the parallel index function electronically, but radar-based parallel indexing retains the advantage of operating independently of GPS and ENC accuracy. The OOW uses it routinely in reduced visibility, in confined channels, or when ECDIS gives inconsistent readings, making it one of the most practical and underrated tools in the navigation toolkit.
Regulatory detail & full definition
Parallel indexing is a radar navigation technique in which an index line is drawn on the radar display parallel to the intended course line, at a distance equal to the planned closest point of approach to a fixed radar-conspicuous target. As long as the echo of the target remains on or outside the index line, the vessel is on track or on the safe side; if the echo crosses inward of the line, the ship has wandered dangerously close. The technique is described in the Admiralty Radar Manual (NP334) and the Admiralty Manual of Navigation and is a standard pilotage and coastal watchkeeping tool.
A single index line set to a coastal headland, lighthouse, or buoy gives a continuous, automatic position check requiring no plotting. Multiple index lines covering successive waypoints can be pre-set before the passage. Modern ECDIS systems can replicate the parallel index function electronically, but radar-based parallel indexing retains the advantage of operating independently of GPS and ENC accuracy. The OOW uses it routinely in reduced visibility, in confined channels, or when ECDIS gives inconsistent readings, making it one of the most practical and underrated tools in the navigation toolkit.
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