Global Positioning System Reference
In-Depth Information
Our intrepid navigator might equally well employ a sextant to deter-
mine his position relative to the coast by referring to two coastal land-
marks, in which case he does not need to estimate his ship's speed because
this method works for a stationary vessel. In figure 7.2 we see that he is at
anchor and measures angles
a
and
b
from a common reference direction
(magnetic north, say) to two landmarks, the beacon and a large palm tree
atop a small hill. On his local chart he marks a
line of position
(LoP) for each
direction from each landmark. The intersection point of these two LoPs
indicates the ship's position.
In general, if a navigator has been estimating his position by dead reck-
oning (perhaps he has been in open ocean for some time, with no ter-
restrial reference points), then he might confirm his estimated position
upon sighting land by making a bearing measurement: the intersection
of one LoP with his dead reckoning track gives his position. Of course,
this method assumes that the landmark he spots was at a known loca-
tion, marked on the navigator's charts. For training purposes, a naviga-
tor might ask his midshipmen to estimate the ship's position by dead
reckoning as it weaves its way through an atoll of Pacific islands and then
FIGURE 7.2.
Getting a fix. A navigator or coastal pilot obtains a fix on his position
relative to a charted coast by measuring two lines of position—the compass direction
to two known landmarks or reference points on his chart. The intersection of the two
LoPs shows the navigator his position on the chart.
