Passing distance and CPA from chart plot
Determining closest point of approach and passing distance from a chart plot or relative motion plot: constructing the relative motion line from successive positions of the contact and measuring the perpendicular distance to own vessel.
Appears on 55% of examsDifficulty 4/551 drill questions
On the paperQ172 / Q35810 questions90% to pass, 1 wrong allowed
Source excerpts#
Bowditch Ch. 13 §1301
Radar Navigation Marine radar transmits microwave pulses and processes the echoes to display a plan-view picture of surrounding targets. Range and bearing to a target give a direct fix.
Bowditch Ch. 13 §1302
Radar Navigation The principal radar range scales used in piloting are 0.5, 1.5, 3, and 6 nautical miles. Short ranges give the best discrimination for collision avoidance; long ranges give the best landfall detection.
Bowditch Ch. 13 §1303
Radar Navigation ARPA (Automatic Radar Plotting Aid) computes target tracks automatically and displays predicted CPA and TCPA. ARPA does not replace bridge watchkeeping; it supplements visual look-out.
Bowditch Ch. 13 §1304
Radar Navigation Plotting successive relative positions of a target on a radar plot or manoeuvring board gives the relative-motion line. Its closest approach to own ship at the plot's centre is the closest point of approach (CPA) — the predicted passing distance — and the time to reach it is the TCPA. A target whose compass bearing holds constant while its range decreases (CBDR) will pass at a CPA of zero, a collision, and calls for early and positive action.
Bowditch Ch. 4 §406
Nautical Charts To plot the true course and distance between two charted points, draw a straight line joining them, then walk a set of parallel rulers from that line to the nearest compass rose and read the true course where the ruler crosses the outer (true) ring. Measure the line's length with dividers against the latitude scale at the working latitude — one minute of latitude is one nautical mile — to obtain the distance. Label the course line C (course) above and S (speed) below.
Practice this topic#
One real exam question on this topic, cited to source. No sign-up.
One minute of latitude on a nautical chart equals ______.
Study this topic#
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The full lesson covers this topic with citation popovers, and there are 51 drill questions behind it.
Drill Passing distance and CPA from chart plot against the real exam.
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