RELATIVE MOTION LINE
(1) Draw own ship's true course from the center to the outer edge of the Plotting sheet, label it "own course" with the symbol (OC) and draw an arrowhead to show its direction,
(2) Choose the range scale that will produce the largest plot. Circle that scale and cross out the remaining three scales.
DETERMINATION OF THE CLOSEST POINT OF APPROACH
(1) Position each contact on the plotting sheet by using its bearing(s) and range(s) as observed on the radar scope. Identify each of the contact's positions as r, ml, m2, m3, etc., and record the time of each of these positions. (Note: Three or six minute intervals will simplify your calculations.)
(2) Construct the Relative Motion Line (RML) by connecting (r) With any later positions (ml, m2, m3, etc.) of the contact. Be sure to extend the Relative motion Line from the final position of the contact well past the center of the plotting sheet and indicate its direction with an arrowhead.
(3) Draw a right angle from the center of the plotting sheet to the Relative Motion Line. Label the point of intersection of the right angle and the RML as CPA. The contact's distance off at CPA is measured from the center of the plotting sheet to the CPA. The bearing of the contact at CPA is found by extending the right angle all the way out to the edge of the plotting sheet and noting, from the compass rose, its direction.
NOTE: Your vessel is located at the center of the plotting sheet anel maybe illustrated by drawing an outline of a ship with its fore and aft axis along the "own ship's course'line". Clearance, if any whether ahead, astern or abeam depends on how the RML runs in relation to "own ship".
If the RML crosses "own ship's" course line, the contact will pass ahead of own ship. The contact will pass ahead of own ship, with clearance, as measured from the center of the plotting sheet to the intersection of the RML and own ship's course line. THIS DISTANCE IS NOT NECESSARILY EQUAL TO THE CPA.
