avoiding collision by eye or radar 151
direction of relative movement (DRM) line. As long as both vessels
maintain course and speed, the vessel will plot down this DRM line.- CPA mark along the DRM line. To fi nd the CPA, measure the closest
point from the DRM to the center of your radar scope. To do this, place
a straightedge (e.g., a batten or tongue depressor) perpendicular to the
DRM line and back to the scope center. Mark this point along the DRM
line. This establishes the closest point of approach, or the CPA mark
(see illustration). - Range and bearing at CPA. Next, run the VRM out so that it just touches
the DRM line. Th is shows a range at CPA of about 1.5 miles. Place the EBL
onto the CPA mark and read the relative bearing: 311 degrees relative.
Th e contact’s magnetic bearing at CPA will be 331 degrees magnetic
(020 degrees magnetic + 311 degrees relative = 331). Label the range and
bearing next to the CPA mark. - Relative speed of contact and time of CPA. Find the relative speed
of the contact. Take the range difference between the 1340 and 1346
marks: 4.8 miles – 3.8 miles = 1 mile. Move the decimal one place to the
right. The relative speed of the contact is 10 knots.
Next, measure the distance from the 1346 mark to your CPA mark. Place your
tongue depressor between the two marks and mark that on the depressor. Use
the VRM or fi xed range rings to estimate distance between the two marks on
the depressor: 3.4 miles.
Th is radar screen is set to a scale of 6 miles and pictures the contact plotting scenario
described in the text.