Radar Plotting

Question 1

What is a radar plot?

Answer 1

A radar plot is the systematic observation of a target to obtain CPA, TCPA, course, speed and aspect of another vessel to assist in collision avoidance

Question 2

How many plots do you need for a radar plot?

Answer 2

A minimum of three at regular intervals because assumptions shall not be made on the basis of scanty radar information (Rule 7 - C)

Question 3

How do you get the CPA of a target?

Answer 3

You extend the O-A line and measure at a right angle from your own ship’s position

Question 4

How do you get TCPA?

Answer 4

Work out how long it took them to travel from O to A. Then you can work out how far it will take them to travel from A to CPA

Question 5

What does the O-A line represent?

Answer 5

This is the way of another and represents the targets relative course.

Question 6

What does O, A and W stand for?

Answer 6

O = The target's original position
A = The target's actual position
W = Your vessel's position in relation to the target

Question 7

What does the W-O line represent?

Answer 7

This is the way of own, your vessel’s true course

Question 8

What does the W-A line represent?

Answer 8

This is the way of another, represents the vessel’s true course

Question 9

How do you determine the aspect of another vessel?

Answer 9

• From A draw a straight line through the centre of the plot
• Take the W-A line through the centre of the plot
• The range between these two will give you your aspect.

Helpful tip = Draw a vessel on the W-A line to give you a rough idea of the aspect to expect