In the series up to this point, I've assumed that you've prepared your maps with lines that point to magnetic north. This differs from true north by as much as 18 degrees east or west, depending on where you are in the U.S. The great virtue of having all the lines drawn in on your chart of map is that your observations with your compass can be matched up with the observations on the map and everything is unambiguously referenced to magnetic north locally.
Topographic maps and nautical charts all display the local magnetic variation. Note that in topographic maps the name is declination, while for marine charts, it's variation. The main distinction has to do with a different meaning of the word 'declination' in nautical applications. In celestial navigation, 'declination' means the angle of a star above the celestial equator, so the term magnetic variation is used in nautical terms. The term magnetic declination is common in land applications, where celestial navigation was rarely practiced, expect by some explorers (e.g. Lewis and Clark).
Below is a map from NOAA of isogonic lines, or lines of equal magnetic declination (variation) for North America in 2010. You can click on the map to get an enlargement. As you probably can see, the magnetic variation around Boston is about 15 degrees west. There's also something called secular variation, which is the change in magnetic variation over time, and it typically quoted on maps as the number of arc-minutes per year of change in variation. If you have an old map or chart, you'll want to account for this.
Magnetic declination (variation) map for North America from 2010 by NOAA. Click to enlarge.
As long as you have drawn the local magnetic north lines and stick to that consistently, it's really straightforward to navigate, however many people will insist in phrasing results in terms of true north. In part, the value of this is on longer voyages where one will be traversing regions where the magnetic variation changes appreciably. Also, when using a Mercator projection, a heading based on true north forms a straight line, even when changing latitude appreciably. For these reasons, true north is often the azimuth of choice.
Here are two stories on this point. I was out doing a kayaking training trip with Shawna Franklin of Body, Boat, and Blade. She was quizzing us on how to take into account magnetic variation. I told her about my trick of preparing charts. She said that this was a bad idea, as she often rolls into town in the evening, buys a chart at a local store. How could one be expected to prepare a chart under those conditions? Well, actually I had done just that the night before.
In another case, I was teaching my system of chart preparations to students in my class at Harvard. After the lecture two guys, one of whom had served in the Marines, and one in the Rangers said that they always used true north and did conversions in the military.
As with long distance sailing, the military has good reason to use true north consistently. This leaves little ambiguity in communications. Instead of quoting locations in latitude and longitude, the US military and NATO use something called the Military Grid Reference System (MGRS), which is derived from a system called UTM (Universal Traverse Mercator). In these systems, locations are quoted in terms of distances - kilometers, meters, etc. This avoids the problem of latitude and longitude insomuch as north-south distances and east-west distances are consistently accounted for. In latitude and longitude - the scales change with latitude, as the lines of longitude converge at the North Pole, introducing a distortion. The price one pays for the MGRS and UTM systems is that one has to become accustomed to the 'patches' of grids that cover the globe.
The upside of MGRS and UTM systems, however, is that many calculations are simplified and a clarity is gained in communications of coordinates. An example is the communication of positions, spotting and correcting artillery fire etc. In this case, a consistent usage of true north aids communications.
Note that in many maps and charts, lines that point true north don't exist or are spaced far apart, so one has to try to get them off the edges of maps as best one can. On the other hand, military maps will typically have lines drawn in to match the MGRS lines.
So, there are situations (and people) where referencing everything to true north is important. This does add a level of complexity. First, one must be careful to communicate an azimuth as either in 'true' or 'magnetic'. Secondly, one has to know how to make the correction. There are four cases of adding and subtracting declination and variation. First, you either add or subtract when going from compass to map, or map to compass, providing you are taking your bearings with "red is in the shed" (i.e. the compass needle lines up with the orienting needle in taking bearings). The second consideration is whether you're in a region where the magnetic declination is east or west. The combination of the two give you four possibilities of adding or subtracting declination, depending on your location and purpose.
There are all sorts of mnemonics for this - perhaps the most famous is "True Virgins Make Dull Companions." The mnemonic I use is the work with is: CADET. The meaning of this is "Compass ADd East for True" - as illustrated in the figure. This is if you're going from compass north - i.e. have a bearing on a compass, and you want to convert the bearing referenced to magnetic north (i.e. from compass) to a bearing referenced to true north, as you might find on a map. In this case, the acronym means that you add the declination if it's east.
The figure below illustrates how to do this for different cases. The arrows point in the direction you want to go. You change sign when you substitute "west" for "east" and you change sign (i.e. subtract rather than add or vice versa) when you are going from true to compass as opposed to from compass to true.
Usage of the acronym CADET for moving from compass to true bearings in both east and west declinations.
So, we covered the first case - in going from a compass to true reading when the declination is east, we add. If we're going from true to compass in a region where there's east declination, the arrow in the figure reverses and we subtract east true to get compass. If we want to go from compass to true with the declination is west, we subtract the declination, and finally when we want to go from true to compass when the declination is west, we add the declination.
Many people find using these mnemonics cumbersome, but I should mention that they're quite common in the military and people are often taught this as a system.
There's yet another approach which I call the 'visual approach'. In this case, bearings are taken and everything is quoted in terms of true north. The actual correction takes place by making sightings where you no longer place 'red in the shed', but rather have the magnetic compass needle deviate to the left or right of the orientation line by the amount of location magnetic variation (declination) as shown in the figure below.
Deliberately offsetting the compass needle by 15 degrees west in taking a bearing so that the orienting arrow is pointing to true north. This is valid only where the declination is 15 degrees west (e.g. in Boston), but the amount should be set for the local declination.
By using this offset method, results can be quoted in true north most of the time.
The is the end of the section on basic compass work. In the next section , I discuss assessing motion, dead reckoning and using vectors.
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