IV-w10 declination
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MAGNETIC DECLINATION
The difference between magnetic north and true north at a particular location.

"A compass, Lou, has a magnetic needle which, when the compass is held level, floats freely over the center of the compass rose. The needle rotates until one end (the black end in this case) points to magnetic north and the other end (the red) points to magnetic south!"
"You do make things difficult! I thought a compass needle points towards North! Now you're telling me it points somewhere else! Oh well, I know when I'm beaten! I better buckle down and find out about... magnetic north!"
"It's pretty straight forward, Lou! The earth acts as if it had a giant bar magnet embedded in it! The points on the earth's surface just over the two ends (or poles) of the 'magnet' are called the magnetic north pole (MN) and the magnetic south pole (MS)! And those are the places to which the ends of the compass needle point! The north end of the compass needle points towards the magnetic north pole and the south end towards the magnetic south pole! (Don't confuse the magnetic poles with the rotational poles, labeled N and S in the diagram!)"
"The magnetic north pole is near to the rotational north pole, but no cigar! The magnetic pole is located on Price of Wales Island in northern Canada (about latitude 73° N, longitude 100° W)!"

 

"So, in most places, there is an angle between the black arrow pointing to true north (the rotational north pole) and the red arrow pointing to 'magnetic north'! That angle is called the MAGNETIC DECLINATION! Notice that the declination can be towards the east or towards the west!"
"How can you find out what the magnetic declination is at a particular location since it varies from place to place?!"


"Not so difficult! If you know the direction of true north from looking at the North Star, then you can point the N on the face of the compass in that direction (to the place on the horizon directly beneath the North Star - the red dot). Then you can see the angle between the black end of the needle (which points to magnetic north) and the N on the compass face!"
"If the compass was level and you were looking down at it, this is what you'd see! The north arrow on the compass rose (the large N) is pointed towards the place on the horizon directly beneath the North Star! That is, towards true north! And the needle (of course) points towards magnetic north! So the magnetic declination for this locality is 45 degrees west!"
"I could do that!"
"On the other hand, if you had a map of an area, you might not have to go to all that trouble! Very often maps show magnetic as well as true north, and give the declination numerically! The magnetic declination for the area shown on the map on the right was about 13 ° W in June, 2000! Do you recognize this place, Lou?"

 

 
"I do, I do! But why June, 2000? Does the declination change through time?"

 
"It does indeed!"
"So how can you keep up with it?"

 
"The easiest way is to get the declination for a place and to discover how it has changed through time is to use the internet! Try it out!

 

 

 Here's a couple of quizzes that'll teach you a thing or two, Lou!

 http://www.ngdc.noaa.gov/seg/geomag/jsp/Declination.jsp

Useable with your zip code

 

 

 

 

   LINK TO QUIZ 1    LINK TO QUIZ 2

Our thanks to Professor of Geology, David Leveson at Brooklyn College , CUNY
for his kind permission to use this copyrighted material.  

http://academic.brooklyn.cuny.edu/geology/leveson/core/linksa/maptop.html

Here is another explanation of Magnetic Declination

Adjusting for Declination

Many people do not know how to properly adjust for declination. Adjusting for declination is important for accuracy in route finding. To have your map and compass speaking the same language, north on your compass must equal true north on your map. To make them equal you need to either add or subtract the declination. Whether you add or subtract depends on if you have an easterly or westerly declination. It also depends on if you are converting a map bearing to a compass bearing or converting a compass bearing to a map bearing.

If you were in an area with a 10 degree west declination it would be very desirable to adjust for declination. To orient a map and compass to true north declination needs to be compensated for.

To orient to true north first set the direction of travel arrow on one of the north/south grid lines of your map. Then move the map and compass together and orient to north. Map and compass are now oriented to magnetic north at 0 or 360 degrees. To orient map and compass to true north you need to add a westerly declination (west is best) or subtract an easterly declination (east is least).

With a 10 degree west declination, to get true north you will add 10 degrees. Set the compass at 10 degrees, keep the direction of travel arrow on the grid, and then orient for north again. Map and compass are now facing the direction of true north. When plotting a course on your map that you will follow on the ground with your compass, you will need to convert the map bearing to a compass bearing. If your map is oriented to magnetic north, or if you got your bearing from a protractor, then you need to adjust for declination. The west is best, east is least rule applies here. A 10 degree west declination will change a 60 degree map bearing into a 70 degree compass bearing. You will use the compass bearing to get you headed in the right direction from point A to point B.

If your map is oriented to true north, then when you use your compass to take a bearing from your map, it would already be adjusted for. It would read 70 degrees which is the direction that you want to travel on the ground. A compass oriented to true north in an area with a 10 degree west declination would have a due east bearing of 100 degrees.

If you are converting a magnetic bearing to a map bearing then the west is best, east is least rule does not apply if your map is oriented to true north. If your map is oriented to magnetic north then a map bearing is the same as a compass bearing. To follow the bearing on the ground it only needs to be properly adjusted for declination. If you're in an area with a 10 degree west declination and your map is oriented to true north, a compass bearing of 10 degrees is 0 degrees on the map. You will subtract a westerly declination. It is the opposite of converting a map bearing to a compass bearing. A 280 degree magnetic bearing will be 270 degrees (due west) on a map oriented to true north with a 10 degree west declination. A 70 degree compass bearing is a 60 degree bearing on the map.

There is a simple way to to illustrate why you need to subtract the 10 degree west declination to get the right map bearing. Orient your map and compass to true north for a 10 degree west declination. Your compass should be set to 10 degrees, the direction of travel arrow should be on the north/south grid line, and the needle should be oriented to north. Now set the compass to 70 degrees and count how many degrees there are between 10 degrees and 70 degrees. It is fairly easy to see that there are 60 degrees. This is why you need to subtract a westerly declination or add an easterly declination when converting a magnetic bearing to a map bearing.