Passing distance and CPA from chart plot

Bowditch Ch. 13 §1301

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

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

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

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

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.