Page 191 - NAVAL SCIENCE 3 TEXTBOOK

I\IARI N E NAVIGATION 197

Fixes and Lines of Posit ion

In plotting a ship's location on a chart, a fLX is an accurate posi-
tion determined without reference to any previolls position, using
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visual, electronic, or celestial observations. A fix position is the
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intersection of two or more lines of posit;oll (LOPs) obtained at the
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same time.
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\ " /d'!l
/' .......... ---

~ An example of a distance arc LOP.

8TACK
measured by using radar, a stnriil1leter (an optical distance-mea-
Two examples of visual gyro bearing LOPs. suring device that operates by measuring the distance of an object
of known height. such as the masthead on a ship), or a se),."tant.
Tn clear-weather navigating near land, Jines of position (lines In selecting objects from which to determine lines of position
along which a vessel mllst be located) are obtained by talUng visual for fixes, the na\'igator must consider the number to be used and
magnetic or gyrocompass bearings to objects or landmarks whose the angle of intersection of their lines of position. The closer LOPs
position is known and printed on a chart of the area. Lines corre- come to crossing at right angles, the less error there generally will
sponding to the bearings are plotted on the chart, extending to the be. Also, for fixes it is best to have at least three LOPs, since each
chart symbols representing the objects or landmarks used. Such acts as a check on the other. If all LOPs cross in a pinpoint or form
lines are called benrillg LOPs. They are labeled with the four-digit a very small triangle, the fix may be considered reliable. If a larger
time of observation above the line. triangle or some other geometric shape is formed, the fix position
is generall)' assumed to be in the center of it.
In summar}', fixes may be obtained by means of the following
b?I combinations of lines of position (the first three are much more
commonly used than the latter three):
,
,
, I '1" '0 or more lines of bearing
A distance arc and a line of bearing
Two or more distance arcs
" A visual range and a line of bearing
A visual range and a distance arc
Two examples of visual range LOPs.
Two simultaneous visual ranges
A special kind of bearing LOP is the viSltal ra1lge. A ship is said
to be on a visual range when two landmarks or navigation aids are
observed in line one behind the other. Such an LOP is considered
one of the most accurate obtainable. Visual ranges are extensively
used when navigating in narrow waters such as canals.
LOPs for fixes can also be obtained by electronic means such
as radar, or by observation of celestial bodies. Each of these will be
discussed in more detail later in this chapter.
A distcmce arc is a circular line of position. \"'hen the distance
from an observer to an object is known, a line of position can be
drawn with the object as the center. The radius is equal to the mea-
sured distance. The entire circle need not be drawn, because the
navigator normally knows the ship's position well enough so that
drawing an arc of the circle is sufficient. The arc is labeled with
A watch officer measures the distance to another ship with a
the time above. expressed in four digits, and the distance below, Brandon sextant-type stadimeter. The stadimeter measures the dis-
in nautical miles (and tenths). The distance to a landmark may be tance to an object of known height, such as the masthead of a ship.
(Emmitt Hawkes)