Parallel Index Techniques in Restricted Waters

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This book is a guide to best practice for parallel indexing in restricted waters.

This guide to best practice for parallel indexing in restricted waters highlights the most common errors and limitations affecting parallel indexing and how these can be safely avoided.

Topics covered include:

  • radar display settings and the limitations of radar
  • speed, rate and radius of turn
  • characteristics of beam width
  • range error
  • heading marker alignment.

This publication underlines the most common errors and limitations that affect parallel indexing and have been observed by pilots of the Corporation of the Lower St. Lawrence Pilots (CLSLP) during their assignments. It should be noted that on any radar there may be other errors and limitations that have not been identified in this book. This book is the end result of observations, experiences and careful monitoring of all aspects of radar navigation in confined waters. It is based on methods that have been tested on the simulator, validated in real life and used by many of the St Lawrence River pilots. The information is useful for all navigators, whether deck officers or pilots.

In marine navigation, the main purposes of radar are to detect any vessels in the vicinity of the ship and any obstacles, such as land masses, in reduced visibility. Radar is also widely used in good visibility.

Navigators are required to understand the limitations and errors that may affect the optimum use of their radar. Navigators should familiarise themselves with the horizontal and vertical beam widths of their radars and understand the effects of these on bearing and range discrimination. Navigators should be aware of the different characteristics and limitations of X- Band (3 cm) and S-Band (10 cm) radar. They should also be aware of when to use different display and pulse settings, depending on the navigational situation. Technical knowledge of the limitations of radar equipment is a requirement for compliance with the COLREGS.

In coastal navigation, a parallel index (PI) line set as a limit to ensure that the ship will not be closer than, for example, 3 miles from the nearest danger is a safe way of monitoring the ship’s progress as small undetected errors will not put the ship in danger. However, in restricted waters, especially in channel navigation, parallel index can be used to ensure that the ship follows the track and cannot be off course by more than a small value (say, 0.03’ (180 ft)). That absolutely entails that all errors and limitations of the equipment have been thoroughly assessed.

The purpose of this publication is to explain how radar can be safely used for navigation. It is not a technical radar manual and it assumes that readers will already be familiar with radar principles.

Navigators should always familiarise themselves with the manufacturers guidance for the radar(s) on their current ship.

Note, in all the examples in this publication, the PIs, ranges and chosen routes are for demonstration only and are not reproductions of actual passages in those waters.



1. Introduction

2. Radar Display Settings for Navigation

3. Limitations of Radar

4. Parallel Index Technique

5. Speed, Rate and Radius of Turns

6. Characteristics of Beam Width

7. Effect of Echo Enhance Function

8. The “Total” Method

9. Range Error (RE)

10. Heading Marker Alignment

11. Gyro Error and Effect on Parallel Index Technique

12. Practical Example of Problem Solving

13. Error Analysis in Reduced Visibility

14. When Not to Use Parallel Index Technique

15. Radar Repeater

16. Examples

17. ARPA: Main Features and Situational Examples

18. ARPA Target Referencing

19. Course Over Ground: ARPA GNSS Discrepancies

20. Radar Image Overlay (RIO), ARPA and ECDIS

21. Radar Mapping Option

22. Some Radar Myths

23. Conclusion


Appendix 1: ECDIS and Radar Answers

Appendix 2: Example of Excessive Range Error in Relation to PI

Number of Pages:
Published Date:
January 2020
Binding Format:
Book Height:
0 mm
Book Width:
215 mm
0.7 kg

Captain Alain Victor, MM