Navigation 101 (Magnetic Compass) Part 2 of 3

Here you can post your User guides and make comments and suggestions for guides available on Airwarfare.

Moderators: MudPuppy, dieg777

Navigation 101 (Magnetic Compass) Part 2 of 3

Postby 100th_Alien on Sun May 14, 2006 3:00 am

Songbirds can do it, whales, butterflies, and pigeons can too, but I'm afraid we humans can't travel a steady course without some form of outside reference. Try it next time you find yourself steaming in a thick fog. Without your compass or a visual reference, you will invariably find your course degrade into some form of circle. Sure, you'll swear you were traveling a straight line, but when you check your wake, you'll see it isn't straight.

Luckily for us the Earth has a magnetic field which is roughly aligned with the north and south poles. Since like magnetic poles repel and opposites attract, a magnet allowed to rotate freely will align itself with these magnetic poles. The modern magnetic compass actually consists of several parallel magnets mounted to a compass card. This assembly pivots around a jeweled bearing and is usually built within a fluid filled container designed to dampen it's movement. These instruments give us the ability to accurately and consistently steer our vessel on a desired course. Naturally, like everything else in life, there's a catch and it's called COMPASS ERROR.

COMPASS ERROR:

Direction relative to the Geographic North Pole is regarded as TRUE direction. Anything that affects our compass reading, that is, anything that alters it from the direction of true north, is called compass error. We need to understand what forces will do this so that we can correct these errors and derive our actual heading.

Variation

As previously mentioned, the magnetic poles correspond roughly with the actual geographical poles. Close, but no cigar. The north magnetic pole is located at approximately 78.9°N latitude and 103.8°W , over 600 miles from the geological north pole. And while your compass doesn't point exactly towards the north magnetic pole, it does point to a location near it.

The problem that's created here is that a compass will point to a direction other than true north, the difference between the two depending where on Earth the compass is. This error is called Variation, and it's the angular difference between true north and magnetic north.

Look at Aliens Fig. M1 to see how variation is different depending on where you are located. The lines are called isogonic lines and are rough boundaries of variation differences.

Image
Bowditch, Nathaniel. American Practical Navigator. Defense Mapping Agency Hydrographic/Topographic Center, 1984

Variation is determined by measuring the angular difference between true north and magnetic north. So, if we are located on Long Island, NY, magnetic north (the direction that our compass points to barring any other errors) is about 14° west of the true, geographic north, so our variation is 14°W. Now let's go to the Aleutian Islands of Alaska. Here magnetic north is about 15° east of true north, and our variation is 15°E.

Image

Let's look at an example of variation. As shown in fig. M2 above, if we look at our compass as we sail next to the Fire Island Light, we know that the local variation is 14° West. That means that all of our compass readings (barring additional errors) are going to be 14° west of the true direction. Say our compass says we are heading on a course of 090° psc (per ship's compass). We are actually heading 076° true. If the compass reads 015°, we're on a course of 001°.

Some things to remember:

1. Another name for variation is declination.

2. An area's local variation can be found within the Compass Rose of your nautical chart. It changes a very slight amount every year due to the slow migration of the Earth's magnetic poles so check the year that your chart was printed and note the annual increase or decrease in variation.

3. Some places have no variation. Other areas have extreme magnetic disturbances, to the point where conventional magnetic compasses are useless. These areas will be marked on charts of the area.

4. Variation affects devices that rely on the Earth's magnetic field to work. Gyroscopic compasses, radio direction finders, and global positioning instruments are not effected by variation.

Deviation

Another force that acts upon your compass to create error is deviation. Deviation is the influence of the immediate environment upon your compass. Being a magnet, your compass will be attracted to (or repelled by) iron bearing metal and other magnets (including magnetic fields created by flowing electricity). Unlike variation, deviation is not constant, it's different in every boat, and it's even different within the same boat, depending on which direction she's sailing. Deviation is measured by the angular difference between the magnetic heading and the compass heading.

The navigator should know what the deviation is on his vessel. While it is beyond the scope of this tutorial to describe the process, most quality compasses can be adjusted to eliminate most, if not all deviation error. What deviation remains can be found and documented on a Deviation Card. This card or graph will list the deviation for various compass courses and is referred to by the navigator when compass courses need to be corrected.

Some more facts about deviation:

a. Objects outside your boat can create deviation. You can bet that if you're 5 feet away from that big green can "11", it'll affect your reading.

b. Don't forget about everyday objects that you take on and off your boat. A common mistake we make as cops is to place the portable radio next to the compass. This is guaranteed to add a huge error to our compass readings.


Next Lesson: CORRECTING COMPASS ERROR

Produced by: Alien
Image
"The four elements: earth, air, water, and fire. Of these, I call your attention to two: air and fire. As pilots we live in the air, but we die by fire."
User avatar
100th_Alien
Flight Sergeant
 
Posts: 77
Joined: Sat Feb 04, 2006 9:36 pm
Location: Xemu Xeno

Return to Guides & Tutorials

Who is online

Users browsing this forum: No registered users and 2 guests

cron