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We have over 800 practice questions on celestial navigation in the online study.
| 1 Introduction | 9 Altitude Intercept | 17 Latitude by Meridian Altitude |
| 2 Corrections to Sextant | 10 Using Position Plotting Sheets | 18 Latitude and Azimuth by Polaris |
| 3 Altitude Corrections | 11 Plotting Lines of Position | 19 Running Fixes |
| 4 Time | 12 Summary of 1 thru 11 | 20 Time of Sunrise Sunset |
| 5 Finding GHA and Declination | 13 Finding Deviation or Gyro Error | 21 Star Identification/Selecting for Fix |
| 6 Assumed Position and Local Hour Angle | 14 Finding Azimuth by 229 | 22 Time Tick Problems |
| 7 Computed Altitude and Azimuth | 15 Amplitudes | 23 Deviation Table Construction |
| 8 Interpolation | 16 Time of Local Apparent Noon | 24 Sample Final Exam |
Finding Deviation or Gyro Error
Two methods will be presented, because you might find either (or both) of these types of problems on the exam. Before we start the hard stuff, though, we will pause for a brief review of how to compare compass and true bearings to find deviation or gyro error.
To pass the examination for your license you had to be reasonably familiar with using variation and deviation or gyrocompass error to convert from true to compass directions and vice-versa, or find the deviation or gyro error when given the other figures.
Three chapters to follow will concern finding the azimuth of the sun or stars to compare with the compass bearing to check the compass. The answer required in those problems is gyro error or deviation.
Every compass, magnetic or gyro has a certain amount of error in it on at least some headings at least some of the time. If it is a good compass, properly maintained, the deviation or gyro error is usually small and often goes unnoticed on short trips. An undetected error of even a degree or two can make quite a difference on a long haul. The upon oceans endorsement makes you eligible for the long hauls, so the Coast Guard requires you to prove your ability to find your compass error offshore. Even if you have your compass adjusted by a professional just before sailing and have an up-to-date deviation table, the table may not be correct a few days later. Deviation changes for many reasons. Gyro error can creep in and disappear without anyone touching the gyro compass. It is a good practice to compare the compass with a known correct bearing frequently. Usually, you find everything is fine, but you might be surprised to learn how often you find some error. It won't get you into trouble if you know about it.
So to review, here is the best known, and the handiest, work form for compass problems.
T is the true direction (course or bearing). The answer you get for Zn from 229, Tables 27 and 28, or the Polaris tables will always be entered next to the T.
V is variation, the part of compass error caused by the fact that the earth's magnetic field doesn't line up with the true or geographic meridians. It is found on the chart. In problems, it is just given, and is entered next to the V. Variation is named East or West. It is the difference between True and Magnetic direction, what a magnetic compass would indicate if you placed the compass where it was affected only by the earth's magnetic field. You solve for this one as one of the steps in the compass problem and put it next to the
M in the work form.
D is deviation, the part of compass error caused by the ship's magnetic field. It is also named East or West. It is the difference between magnetic and compass. This will be the final answer you're working for unless the question asks for:
Gyro error (G.E.) is the difference between the true direction and the direction indicated by the gyrocompass.
C stands for compass, either the magnetic or gyrocompass.
Follow through this demonstration problem:
PROBLEM: You make an evening observation of Polaris bearing 003.5˚ PSC. Variation was 40˚20' East. (A full scale problem would give you the data necessary to solve for the azimuth of Polaris, but for this one we'll just give you that solution) Zn = 001.2˚.
Required: The deviation.
Make the work form and fill in the information you know:
The answer wanted is deviation. Before you can get that, you have to fill in M for Magnetic, so you will have to work the form.
Working down the form, you follow the rules on the left side with the down arrow: If it's named West, add it. If it's named East, subtract it.
Notice how we expressed Zn 00l.2˚as 361.2˚and compass bearing 003.5˚ as 363.5˚ to make the subtraction easier.
Deviation is the difference between magnetic and compass. 363.5˚ minus 356.9˚ equals 6.6˚. The only thing left to do is name deviation east or west.
Check out one more demonstration. This time with a gyrocompass.
PROBLEM: The rising sun on the visible horizon bore 112.7 PGC on (here you would get the time, date and other information necessary to find the Zn yourself. We'll pretend you've done that and find the azimuth is 115.3.
Required: The gyro error.
The same work form will work. Just drop the V, M, and D and substitute gyro error, which has nothing to do with magnetism.
Fill in the form:
The name of gyro error is found the same way you found the name of deviation. Working down, to get from 115.3˚ to 112.7 ˚ you would have to subtract the 2.6˚, so it must be named East.
Starting at the bottom and working up, you would have to add, and it still checks out. The best way to double check any compass problem is to work it the second time in the opposite direction.
Last edited on 25-Sep-2009 04:19:02 -0500
