2008 REED’s East Coast Edition - Navigation
clearly shown on your chart—for ex- when a significant time interval separates the
ample, a point of land in line with a water observations. In fact, on a fast-moving ves-
tower. sel you should be suspicious if three or more
• It’s essential to relate precisely the object LOPs do intersect at a point. Normally you’d
in view to its position on the chart. For expect them to form a small triangle.
example, your observation of the direc- It’s sometimes prudent to think of LOPs as
tion to the water’s edge of a gradually giving you an area of possible positions rath-
sloping point of land can vary with the er than a definitive fix. You can visualize or
height of tide. plot that area by treating each LOP as a cone
Soundings
or band bordered by lines that represent the
extent of potential error. If, for example, you
LOPs are commonly thought of as straight
judge that a bearing could be off by no more
lines based on bearings. To some degree,
than 3° on either side, then you could safely
you can treat lines based on soundings in
assume that you’re somewhere within an arc
the same way. Especially where bottom con-
of 6° centered on the observed bearing, as
tours form a regular pattern—as off the east
illustrated by the shaded area in Figure 2.
coast of Florida—or where depths change
abruptly, soundings can provide a reliable
indication of position. So, for example, one
can proceed safely on long stretches of the
east coast of Florida by staying on, say, the
six- fathom curve, steering more toward
shore when the depth exceeds six fathoms,
and away when it rises above that depth. Or,
when crossing the Gulf Stream toward the
Bahamas, you’ll know precisely when you’ve
Figure 2
entered the shallow banks by noting when
you abruptly come “on soundings.” Where two or more such shaded areas
Cruisers in the Bahamas, as in other places, overlap is your area of possible positions.
don’t even need depthsounders to use “sound- Note that the shape of the area depends on
ings.” They learn to estimate depths quite ac- the angular relationship among the objects
curately by observing changes in water color. observed. The shape for two objects 90°
Soundings should be used routinely to apart is more compact than for two that are
confirm the plausibility of positions based close together, or nearly reciprocal, in bear-
on other LOPs. And in dense fog, they may ing. So, too, the shape is more compact, and
provide the primary basis for knowing your useful, when three LOPs are 120° apart. You
position, in the absence of electronics. should be guided by these results in choos-
Fix Accuracy
ing objects to observe. By doing so, you can
minimize the effects of errors in LOPs.
Only in the unlikely event that you’re locat-
ed simultaneously on two ranges can you have
The Running Fix
complete confidence in a fix based on only When only one known object is visible and
two LOPs. You need at least a third LOP, or you can’t get both a range and bearing to it,
preferably more, to determine whether you’re you can take bearings to it at different times
really at the intersection of the first two. If to obtain a running fix. This is the procedure:
additional LOPs intersect at the same point, • Take the first bearing and note the time;
then you’re in good shape; if not, and the plot the position line on the chart and
three or more LOPs form a sizable triangle, mark it with its direction and the time.
then you’d better check for errors in the LOPs. • When the bearing has altered enough to
Common sources of errors in LOPs include make a good crossing angle with the first
failure to take compass deviation fully into ac- position line, repeat the procedure.
count, failure to precisely locate the object ob- • Advance the first line by moving any
served on the chart, and the inherent difficulty convenient point on it by the “run”—
of using a steering compass to take bearings. the course and distance made good over
Even accurate LOPs will intersect in a triangle the ground during the time interval
between observations.
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