Magnetic Declination
Three Norths
There are 3 different Norths on a Map.
True North
This is where the North Pole is.
Magnetic North
This is where your compass points to
The magnetic pole is around the Canadian Arctic and is slowly moving towards Siberia
On the West Cost of the US, a compass will point off to the east of True North
On the East Cost of the US, a compass will point off to the west of True North
Grid North
This is where the lines on your map line up North and South
Making square, 2D maps for a round planet is challenging and the grid lines don't always line up perfectly with longitudinal lines for the earth (which is also not perfectly round)
The three different norths point in the same general direction for the most part. That said, Magnetic North can be off a lot, especially as you travel north or south towards the magnetic poles. Here in Washington State (depending on where you are in the state), a compasses needle points about 15° to the east for True North. If you were to travel 100 miles towards Magnetic North with a 15° Magnetic Declination, you would be 26 miles off from a point 100 miles towards True North.
Magnetic Declination
A compass does not point at the North Pole. It lines up with the magnetic field of the earth.
Simplified, a compass in the lower 48 points more or less at an area in Northern Canada. If you are on the West Coast, your compass will deviate to the east. If you are on the East Coast, your compass will deviate to the west. If you are in Louisiana, it points more or less north.
It is also important to note that this points moves more than 30 miles a year, so older maps will likely have very outdated declination angles. Here in Washington State our Magnetic Declination changes about a 1/10 of a degree W each year.
Grid Declination
The world is round, so not all maps can line up exactly north and south. This becomes more of an issue as you more closer the the poles where the lines of longitude converge.
This is important when determining magnetic declination but declination is usually measured between True North and Magnetic North. You may need to add or subtract the Grid North from your conversion depending on how your map is setup. This becomes more of an issue when using older maps were the printed Magnetic Declination is out of date and you are using up to day Magnetic Declination instead of what is referenced on the map.
Some math may be required.
Declination Diagram
Most quality topographical maps will have a Declination Diagram in the Map Legend. This shows the relationship between True North, Grid North and Magnetic North (generally at the time of printing).
In the Diagram shown:
The difference between True and Grid North is 0°36' (0.6°).
The difference between True and Magnetic North is 15°41' (15.7°).
To determine the difference between Grid and Magnetic North, subtract 0°36' for 15°1'
We round to the closest degree = 15°
If you are using up to date Magnetic declinations angles, then subtract 0°36' to get Grid North
Note that Magnetic Declination changes over time. Maps that are decades old will have Declination Diagrams that are off by several degrees.
Current Declination
Declination changes over time and should be checked annually. Write the declination on your maps and adjust the declination on your compasses if possible.
You can find up to date information here: NOAA Declination Calculator
Converting Between Grid and Magnetic North
Unless you are using a specialty Orienteering Map, a modified map or a compass with an adjustable declination, you will need to do some conversions.
Easterly Declination (Positive Declination)
In Washington State, we have an Easterly Declination. Magnetic North points around 16° to the east as shown in the image. Remember that this angle varies depending on where you are at in the world and changes every year.
To convert a Magnetic Azimuth to a Grid Azimuth ADD the angle
Use this when you record an angle with your compass and want to draw it on a map
To convert a Grid Azimuth to a Magnetic Azimuth SUBTRACT the angle
Use this when you take an angle off your map and want to find that azimuth with a compass
Example:
Difference between Grid to Magnetic should include the difference between True and Grid.
15°41' - 0 36' = 15°5' or about 15°A 55° Grid Azimuth (55°G) = 55° - 15° = 40° Magnetic (40°M)
Westerly Declination (Negative Declination)
Many places in the world have a Westerly Declination, such as the East Coast of the US. The needle on your compass will point to the east of Grid North. Remember that this angle varies depending on where you are at in the world and changes every year.
To convert a Magnetic Azimuth to a Grid Azimuth SUBTRACT the angle
Use this when you record an angle with your compass and want to draw it on a map
To convert a Grid Azimuth to a Magnetic Azimuth ADD the angle
Use this when you take an angle off your map and want to find that azimuth with a compass
Plotted points and direction of travel.
Using a protractor to determine Grid Azimuth for travel. Convert this to Magnetic by subtracting the Grid/Mag angular differnce.
Map Protractor
In order to make precise measurements on a map, you really need a good Map Protractor. For certain tasks, such as Resection, Intersection, Surveying and calling in artillery, this tool is a required.
For Backpacking and most Scout type outings, we can get away with using a Baseplate Compass, which can serve as an adequate Map Protractor for shorter measurements. Trying to measure angles with a compass with a metal or opaque plastic case is going to be much more difficult and far less accurate.
If you carry a military compass, you should also carry a transparent protractor.
Getting the Rules Straight
It's easy to get the rules mixed up and if you do, then you double the declination error. So instead of veering off 16° to the right direction, you veer off 32° away from the right direction - that's HUGE! A 1 mile trek with that error puts you over a half mile off course!
There are a few tricks that people use to help remember whether you add or subtract with a westerly or easterly declination.
The best trick is to just write down the rules on your map with up to date declination angles.
You'll always have your rules with you and don't need to rack your brains about them.
Add Left, Subtract Right
When converting from left to right, you add the angle
When converting from right to left, you subtract the angle
Major General
General (Grid) to Major (Magnetic) is a demotion
Subtract angle when converting from Grid to MagneticThis only works with Easterly Declination (West Coast)
This one works in Washington State
This works with with the USGS Tenino Washington map the Military used for navigation training
It doesn't work for East Coast maps used for navigation training for the Military
Can Dead Men Vote Twice At Elections
Compass +/- Deviation → Magnetic +/- Variation → True (Adding Easterly)
Used by Mariners and Pilots where the metal of their ship/aircraft has a magnetic field resulting in Deviation
Variation is a magnetic interference common to all vessels
The angular difference between true north and magnetic north
Depend on where you are in the world - aka Magnetic Declination
Deviation is a magnetic interference unique to the vessel itself
The difference between the magnetic heading indicated by the compass and the actual magnetic heading of the ship/aircraft
Different on every ship/aircraft
Different at different headings (Recorded at 16 cardinal directions - N, NNE, NE, etc.)
Easterly Declination ADD
Compass Bearing + Deviation (Error of Magnetic Field of Ship/Aircraft) = Magnetic Bearing
Magnetic Bearing + Variation (Magnetic Declination) = True Bearing
Westerly Declination SUBTRACT
Compass Bearing - Deviation (Error of Magnetic Field of Ship/Aircraft) = Magnetic Bearing
Magnetic Bearing - Variation (Magnetic Declination) = True Bearing
How is this useful?
Remember that "Twice At Elections" means to ADD to Magnetic to get True North
West is best, East is least
Easterly Declination
Subtract the declination from the true reading to obtain the magnetic reading
Magnetic = true - easterly declination
Westerly Declination
Add the declination to the true reading to obtain the magnetic reading
Magnetic = true + westerly declination
Mississippi Rule
For locations east of the agonic line (zero declination), roughly east of the Mississippi:
The magnetic bearing is always BIGGER.For locations west of the agonic line (zero declination), roughly west of the Mississippi:
The magnetic bearing is always SMALLER.
Rhyme
Grid to Mag then Add
Mag to Grid to Rid
This only works with Westerly Declination (East Coast)
Does NOT work for Washington State