Page 63 - Collision Avoidance Rules Guide

bearing for each change of range of 1 mile as a vessel approaches or
recedes, assuming that neither vessel alters course or speed. It will be
seen that even if the closest approach distance is over one mile the
rate of change of bearing will be relatively small at the longer ranges
indicated. Small errors in ranges and bearings taken in the early
stages of an encounter, or inaccurate plotting, are therefore likely to
have an appreciable effect on the assessment of risk of collision.
Consider, for example, a target approximately a point on the star-
board bow, the actual bearing of which remains constant. If bearings
are taken when at distances of 12 miles and 10 miles away, and an
error of - 1" is made in the first bearing followed by an error or + 1"
in the second, the target would appear from the plot to be on a paral-
lel and opposite course, and might be expected to pass clear to star-
board with a nearest approach of over 2 miles. On the other hand, if
an error of + 1" is made in the first bearing and - 1" in the second, it
would appear that the target is crossing and should pass clear to port
at a distance of over 2 miles.

12m HM 101"

The example serves to illustrate the danger of attempting to assess
whether risk of collision exists from a small number of observations
taken at long range. Errors in bearings of 2 1" are not unlikely when

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