Make your own free website on Tripod.com

picture1.jpg

Magnetic Declination of the MAP

Home | About Us | Contact Us | DPM Activities | Basic Mountaineering Course (BMC) | DPM Profiles

There are 3 different Norths that we need to know about:

  • True North - This is the direction towards the North Pole. Lines of longitude meet at the North Pole.
  • Grid North - This is the direction of the North-South Grid lines on the map. As grid lines are parallel lines not all of them can point to the North Pole. So there is a small difference between grid north and true north because the earth is a sphere and the map is flat.
  • Magnetic North - This is the direction that the compass needle points. The position of the North Magnetic Pole varies a bit from year to year. Currently it is somewhere near Bathurst Island in the NWT (or may be Nunavut now).
The Magnetic Declination at a point on the earth's surface is the angle between grid north and magnetic north.

Somewhere on the border of most maps there is a set of arrows to indicate the relationship between the 3 norths at a particular point on the map. Usually, there will also be a narrative like:

Magnetic North was 20 degrees east of grid north in 1976 and this angle is decreasing by 5' a year. (1' means 1 minute or 1/60th of a degree. There are 360 degrees in a circle.)

Some examples of magnetic declination:

magnetic declination examples

Knowledge of the magnetic declination is needed in order to use the compass correctly to either find your way or identify distant mountains. Most maps and compasses have 360 degrees in the circle. However in Norway they use 400 degrees to the circle - so beware!

Taking Compass Bearings

From Map to Ground

This technique is used for finding out which way you want to go from a known point - used for navigation in treed areas and in bad visibility conditions above treeline.
  • Using a compass with no built in adjustment for magnetic declination:
    1. Place a corner of the compass that is closest to the string hole on your known position on the map.
    2. Align the long side of the compass along the direction you wish to travel.
    3. Rotate the circle part of the compass so that the North mark on the compass circle and the lines within it align with the north-south grid lines on the map. N.B. The position of the compass needle and the orientation of the map are irrelevant!
    4. Read off the number on the circle part of the compass by the arrow or mark towards the direction you wish to travel. (This is the angle between Grid North and the direction you wish to travel)
    5. Convert this angle to a magnetic bearing by using the diagram of the 3 norths on the map: Place the compass on the norths diagram with the N of the compass circle pointing to grid north on the diagram. Rotate the compass circle the required number of degrees towards magnetic north on the diagram.
    6. Put the map away and align the compass so that the North end of the compass needle points to the North mark on the compass circle.
    7. Walk in the direction to the arrow or mark on the compass - keep watching that needle!

  • Using a compass with a built in adjustment for magnetic declination:
    1. Make sure the declination marker arrow is set correctly for the map you are using.
    2. Place a corner of the compass that is closest to the string hole on your known position on the map.
    3. Align the long side of the compass along the direction you wish to travel.
    4. Rotate the circle part of the compass so that the North mark on the compass circle and the lines within it align with the north-south grid lines on the map. N.B. The position of the compass needle and the orientation of the map are irrelevant!
    5. Put the map away and align the compass so that the North end of the compass needle points to the declination marker arrow.
    6. Walk in the direction to the arrow or mark on the compass - keep watching that needle!

From Ground to Map

This is used for:
  1. Identifying unknown objects or mountains from a known point or
  2. Finding your unknown position on the map from 2 or more known visible objects.
Instructions for (i) below:
  • Using a compass with no built in adjustment for magnetic declination:
    1. Point arrow in body of compass at unknown object.
    2. Rotate the circle part of the compass so that the north end of the compass needle points to the north mark on the compass circle. (Keep compass body pointing at the object)
    3. Read off the number on the circle part of the compass by the arrow or mark on the compass that was pointing towards the object. (This is the angle between Magnetic North and the direction of the unknown object)
    4. Convert this angle to a Grid bearing by using the norths diagram on the map. Place the compass on the norths diagram with the N of the compass circle pointing to magnetic north on the diagram. Rotate the compass circle the required number of degrees towards grid north on the diagram.
    5. Get the map out. Place a corner of the compass that was nearest you when you looked at the object, on your known position on the map. N.B. The position of the compass needle and the orientation of the map are irrelevant!
    6. Keeping that corner of the compass in place align the long side of the compass so that the North on the compass circle and the grid lines in the circle align with the North-South grid lines on the map.
    7. The unknown object is somewhere on the line of the long side of the compass in the direction of the arrow on the compass body. You will need to extend the line beyond the compass. The object may not be on your map!

  • Using a compass with a built in adjustment for magnetic declination:
    1. Make sure the declination marker arrow is set correctly for the map you are using.
    2. Point arrow in body of compass at unknown object.
    3. Rotate the circle part of the compass so that the north end of the compass needle points to the north end of the declination arrow of the compass. (Keep compass body pointing at the object)
    4. Get the map out. Place a corner of the compass that was nearest you when you looked at the object, on your known position on the map. N.B. The position of the compass needle and the orientation of the map are irrelevant!
    5. Keeping that corner of the compass in place align the long side of the compass so that the North on the compass circle and the grid lines in the circle align with the North-South grid lines on the map.
    6. The unknown object is somewhere on the line of the long side of the compass in the direction of the arrow on the compass body. You will need to extend the line beyond the compass. The object may not be on your map!

 

Back to BMC

  

Copyright 2007. This site is build and maintained by J. Tanega. If you have further question email at joepz.tanega@hotmail.com

eXTReMe Tracker