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Lines of Position and Fixes
Defined
A line of position (LOP) is a line established by observations or
measurement on which a vessel can be expected to be located. The
concept of a LOP is extremely important in piloting. From a single
LOP, one can safely assume that the ship is located somewhere along
that line.
A LOP may be straight (for bearings) or curved (for ranges). To obtain
a high degree of accuracy when fixing the ship’s position, you must use
three or more LOPs.
Accuracy: Factors such as chart errors, human limitations, and
equipment errors may affect the accuracy of a LOP. The accuracy of
any single LOP can be checked by comparison with two or more LOPS
that are taken simultaneously. A bearing will be suspect if it plots away
from two other LOPs.
Fixes
A fix is the point of intersection of two or more simultaneously obtained
LOPS indicating your ship’s exact position on the chart. The accuracy
of a fix obtained from two LOPS is almost always questionable.
Since
we have already stated that accuracy is of the utmost importance in
piloting, you must always strive to obtain three or more lines of position
for an accurate fix.
Labeling LOPs
Any single LOP that is obtained must be labeled with the time that it
was obtained. This is necessary if it is to be in a running fix. An
unlabeled LOP can mistakenly be used and become a source of error.
In the practice of piloting, single LOPS are not common because
bearings on objects are taken at the same time by the bearing takers.
The fix resulting from these bearings is labeled with the time the
bearings were taken.
Selecting
The angle between selected objects is the most important factor to
Landmarks for
consider when selecting objects to fix the ship’s position from. For
LOPS
three simultaneous LOPS to provide the best fix, they should be located
120° apart. This is often impossible in practical application. Normally,
the assistant navigator or plotter will select possible objects to obtain
bearing and the resulting fixes from. Use of conspicuous landmarks is
always desirable. Lighted towers, water tanks, and buildings are some
possibilities.
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