Magnetic declination adjustment
Translating a map’s azimuth based on True North to real-world azimuth using a magnetic compass requires use of Magnetic Declination. That's because the baseline (0°) is different in True and Magnetic azimuths.
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Declination is either East Declination (where the Magnetic North points to the East of the True North) or West Declination (Magnetic North points to the West of True North).
On a map using True North as your starting point, the target position would be found by starting at True North 0° and turning clockwise 60°. Then you'd be facing the object and you can just start walking that direction.
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But suppose you cannot find True North, but you can find Magnetic North. Also suppose at your location, there's a 20° East Declination. That means that Magnetic North is 20° East of True North. If you start by pointing at Magnetic North 0° which you can easily find in the field (with your compass), and turn towards the same object, you only have to turn 40° instead of 60° because you started your turn at a different point.
That's the way you use your position's magnetic declination to adjust your TRUE azimuth to MAGNETIC azimuth. Using your magnetic compass and your new magnetic azimuth, you can now point to the target position in the field without knowing where True North is in the field.
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A simple way to remember this is to use the phrase “East is least (subtract), West is best (add)” to go from MAP to COMPASS. So if you’re at a location with an East declination, subtract your declination from your True Azimuth to get your Magnetic Azimuth. If your location has a West declination, add your declination to the True Azimuth to obtain your Magnetic Azimuth to be used on your compass.
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Can you draw out this situation if your current declination is 20° WEST DECLINATION? What's your magnetic azimuth? Answer is below*.
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East declinations occurs west of the agonic line.
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West declinations occurs east of the agonic line.
Going from the field (compass) to map
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In the previous section we talked about going from the Map (True) azimuths to the field (magnetic) azimuths. Suppose you site an object on your compass and want to translate that to the map, like what you would do in triangulation.
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In that case, you would do the opposite of what you just did. West declination is considered “-” and East declination is “+”. Simply add the declination to your compass reading to obtain the True bearing (based on True North).
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The Easy Way!
I saved the best for last.
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All that conversion seems like a lot to deal with, doesn't it? Fortunately, some compasses have a built in correction for Magnetic Declination.
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If your compass does have an adjustment, always check it prior to using the magnetic needle part of the compass in the field. Make certain it is set properly to the right direction (East vs. West) and amount (number of degrees) for your current location! If not, you'll need to adjust it before using it. Ask coach how to do that.
Suunto MC2 magnetic declination adjustment. Currently set to 0°. (Seen from the back side)
Notice the small screw at the 1:00 position. That's the adjustment.
Suunto MC2 magnetic declination adjustment. Currently set to
16° East Declination.
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Notice the Orienting Arrow is offset from the Orienting Lines. The offset here compensates for the difference between magnetic and true norths.
Using this adjustment, all of the conversions between magnetic and true azimuths is done automatically.
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The magnetic declination adjustment on the MC2 mirror compass only moves the red Orienting Arrow; the "shed" that you put "Red" into. It changes nothing else. So you can still use the Orienting Lines (unaffected by the magnetic declination adjustment) to determine True Headings on a map. Once you dial in the magnetic declination adjustment, you set and read TRUE HEADINGS under the compass index line and there's no need to convert! (Essentially, you're moving the Shed when you put Red in it and that corrects for declination.)
If you have a map of the area you're currently at and you have the magnetic declination adjustment dialed-in, you can just use the compass' Orienting Lines to get a bearing on a map, then immediately use it to shoot the bearing in the field. You can do the same in reverse if you want to triangulate your position. Either way, magnetic declination adjustments allow you to not have to do the math in the field.
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Using a Topographic Map and Compass
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Determining the azimuth from where you're at to another location. It's best to use a baseplate or mirror compass for this.
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1) Determine your starting and ending point accurately.
2) Place a ruler between the two. Hold it steady.
3) Place your baseplate compass squarely against the ruler so that the DIRECTION OF TRAVEL ARROW is pointing towards your destination.
4) Rotate the compass housing so that the orienting LINES on the compass are parallel to a nearby North/South line of longitude on the map. Make sure the compass North is aligned with map North. Note, as long as your ruler is squarely on the start and end points, you can slide your compass anywhere along ruler to overlay it against a line of Longitude.
5) The heading on the compass dial at the DIRECTION OF TRAVEL ARROW is your True Heading.
6) As long as the Magnetic Declination is set as described before, you should be able to just pick up your compass and shoot the bearing to location you just mapped. This time, in the field, you'd put red in the shed to orient the compass.
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*Answer: Magnetic azimuth is 80°.
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Q: Is the compass below set correctly to 20° E declination?
REMEMBER: North is just a starting point (0°) and your azimuth is how much you turn clockwise from 0° your target position (destination).
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Consider the following example:
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You are currently at the star and you want to go to an object at the diamond as shown.
