The true north pole and the magnetic north pole are not in the same place. Since they are offset from each other there are two different angular measurements we can use. Our choice depends on what type of navigation we are doing. In celestial navigation we would use true north but for navigating around the bay or between islands we'd probably use magnetic north. Magnetic has an advantage because our magnetic compass aligns itself with this field. However, we will still need to be aware of true north. It is good practice to stick with one or the other and label them correctly when plotting a course on a nautical chart. If we wanted to sail a compass heading of 035°, we could use a capitol 'M' after it to indicate it is magnetic.
In the Google Earth image at left the green pushpin is true north while the red pushpin is magnetic north. They are actually over 500 miles apart. Variation is what we term the angular difference between them. See the compass rose below for a further explanation.
The outer rose (circle). This represents true bearings on the chart where '0', at the top of the rose, always points to true north. True north is often represented by a star icon, a symbol of the north star, also known as Polaris. True north represents the axis about which the Earth rotates on a daily basis (see Latitude & Longitude). A line drawn through '0' and '180' will always point to the north and south poles. A line drawn through '270' and '90' will always point east and west and be parallel to lines of latitude on the chart.
The inner rose (circle). This represents magnetic bearings on the chart where '0', in the upper part of the rose, points to the magnetic north pole at the time the chart was printed. Since the magnetic poles are moving and shifting we need a way to be able to update the chart to know where the magnetic pole is now. This is accomplished by reading the inside of the rose where an annual increase or decrease is written. In the above rose we can see that 8' (the slash after the 8 represents minutes of degrees) must be subtracted from the variation every year after 1985 in order to be accurate. For an explanation of degrees & minutes see Latitude & Longitude.
Variation. This is the difference, in degrees, between true and magnetic. Variation can be east or west. On the above compass rose we can calculate the variation visually by drawing a straight line, starting from the the center then going through the inner rose and continuing across the outer rose. The difference between the two is variation. We can also read, inside the rose, the variation (abbreviated VAR) in 1985 for this chart is 4° 15' West. Typically, on the east coast of the US variation is westerly while on the west coast it's easterly.
Compass. The ship's compass will align itself with the magnetic field it 'sees'. This may or may not be the same as the Earth's magnetic field. The compass will be accurate as long as there are no additional magnetic influences. For instance you may have just put down a hand held radio next to the compass and the permanent magnetic in the radio's speaker will create a magnetic influence on the compass. Another compass such as a hand bearing compass placed nearby will also have an influence. Deviation is the term we call the difference between magnetic and what the compass actually reads.
Deviation. Is stated the same way as variation; that is, by degrees east or west. Deviation is not as simple as variation however since deviation can and does change on different compass headings. We would normally have a deviation table made up for our ship's compass so that we would know how much deviation there is on any given heading. A quick and easy way to check to see if our compass has any error is by comparing its reading with that of a hand bearing compass. If we take the hand bearing compass to a different part of the boat and then take simultaneous readings on the same object we should get a pretty good idea of the amount of error we have.
Converting between true, magnetic, and compass. There are times when we need to convert bearings and headings from true to magnetic or magnetic to compass. For instance, the set of a current is always given in degrees true. This is because, as you will recall, magnetic headings change over time whereas true bearings never change. Also, we may have a significant compass error (deviation) and need to correct for that.
The easiest way to convert between true, magnetic, and compass is mathematically by adding or subtracting the appropriate amount. For example, if we are sailing where the above illustrated compass rose is accurate, assuming no compass error, and our goal is to sail true north this is the way we would figure out the appropriate compass heading to sail: True north is 0° and the variation is 4° west (we round it off to the nearest whole degree) then the magnetic heading to sail would be 0° + 4° = 4°. So, we would sail a compass heading of 4°. Or, more appropriately we would round 4° to the nearest 5° increment and sail that course as our ship's compass is graduated in 5° increments. The rule is: when we are converting from true to magnetic we add west variation but subtract east variation. You can verify this by drawing a line; Start from the center of the the rose and continue out through true north then simply read off the magnetic heading number the line crosses over.
Now, what if we wanted to convert from magnetic to true? We would do the opposite mathematical operation from the previous example. We would subtract west variation and add east variation. To make this a little easier to digest we can put it all together into a brief, more easily memorized format:
Add Westerly (subtract easterly)
The rule to follow is when going down the above list as in; true to magnetic or magnetic to compass we add westerly variation or deviation and subtract any easterly variation or deviation. When going up, or in the opposite direction, as in; compass to magnetic or magnetic to true we subtract westerly and add easterly.
An easy way to memorize the formula is to come up with a mnemonic that includes the letters T, V, M, D, C, A, W. A favorite one is: True Virgins Make Dull Companions, Add Whiskey.
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