Page 195 - NAVAL SCIENCE 3 TEXTBOOK

I\IARINE NAVIGATION 201

Plotting a ship's DR track from one fix to the next is a contin- 12. Using an atlas or other reference book, locate the following
uous process while under way. A constant check on approximate places ill terms of their latitude and longitude, in degrees and
position helps the navigator to locate an assumed position for minutes:
celestial obsen'ations reasonably close to the ship's actual position. Washington, D.C.
At sea, navigators using celestial navigation will obtain and plot
Chicago, Illinois
fixes at least every morning, noon, and evening. In piloting waters,
San Diego, Cllifornia
the navigator and quartermasters will normally be on the bridge
plotting numerous fixes as often as every three minutes whenever Honolulu, Hawaii
usable navigation aids come into sight. Colon, Panama
As mentioned previously, electronic plotters incorporating Gibraltar
continuous fix updates from GPS or loran, and projecting current
Baghdad, Iraq
ship's position and the DR track onto an electronic chart projec-
Clbkyo, Japan
tion on a computer screen, are becoming ever more widely used
in marine navigation. For many navigators such plotters elimi- Sydney, Australia
nate the need to maintain a manual DR plot during normal opera- 13. How are distances measured at sea? Compare a land or stahlte
tions. The know-how to do a manual plot is still a necessar)' skill mile with a nautical mile.
for every navigator to have, however, in the event that a power loss, 14. How is nautical direction or course measured?
equipment failure, or damage renders the electronic plotter unus-
15. \Vhat are the true bearings of the cardinal points, N, E, S, \V?
able. Even when electronic plotters are in lise, therefore, Illany nav-
16. Compare and contrast a true bearing, a magnetic bearing, and
igators keep a manual DR plot on a paper chart as a backup and
a relative bearing.
for practice, especially when in piloting waters.
17. If a ship is on course 050CYf, and a lookout sights an object on
o
the starboard beam at 090 R, what is the true bearing of the
object?
STUDY GUIDE QUESTIONS 18. If a lookout sights a merchant ship at 285'R, forward of the
port beam, and own ship's course is 1 35"T, what is the true
1. \Vhat is navigation?
bearing of the contact?
2. V/hat is a chart? How does it difter from a map?
19. \·Vhat does the hydrographic information on a chart consist
3. \·Vhat is the terrestrial sphere? of!
4. \·Vhat are the imaginary lines that run through the poles 20. A. \Vhich projection is used for almost all nautical charts?
around the Earth?
B. \Vhere is the greatest distortion on this kind of projection?
5. \·Vhat name is given to the imaginary line formed by a Why'
horizontal plane passing through the center of the Earth,
21. \·Vhat is meant by the scale of a chart?
clltting ever)' meridian in half?
22. \·Vhat tool is used to determine distance on a chart with a
6. \Vhat is a great circle?
linear scale?
7. \·Vhat is the shortest distance between two points on a globe?
23. A. \Vhat are soundings?
8. What are the lines going around the Earth parallel to the
13. How are they shown on a nautical chart?
equator called?
24. \·Vhat is a navigational fix?
9. A \·Vhat is the circumference of a circle?
25. \·Vhat arc lines of position?
13. How many degrees does it have?
26. A. What is piloting'
10. Into what units may degrees be divided?
B. \Vhen is it used?
11. A. \Vhere is the reference place for the prime meridian?
27. What kind of shipboard device is used to determine depth of
B. \·Vhat is the meridian exactly opposite the prime meridian
water?
on the other side of the globe catied?
28. A. \·Vhat are some of the advantages of radar?
C. \"'hat two hemispheres does this great circle line create?
B. \·Vhat is radar's chief disadvantage?