Above picture road at North Bay, Canada
GRID MAGNETIC ANGLE
Grid Magnetic Angle
This information is found at the top centre of the map sheet, and in the technical section, and is required so the grid magnetic angle (GMA) can be calculated for the current year.
It is used when converting grid and magnetic bearings.
On the 1:25,000 Explorer map this information is found in the general information section, under the sub-heading North points.
 | Information Required to Calculate the GMA The example opposite reads as this: Grid Magnetic Angle 02' 49' (50 mils) W of Grid North July 2004 at centre of sheet. Annual Change about 11' (3 mils) E. |
The angles between these Norths are defined as follows:
- Magnetic Declination: The angle between magnetic north and true north at any one point.
- Convergence: The angle between grid north and true north.
- Grid Magnetic Angle: The angle between grid north and magnetic north, which is always measured east and west from grid north
Annual Magnetic Change
The direction of the magnetic north relative to true north, or grid north at a given point on the Earths surface changes continuosly with time. For convenience the rate of change is expressed in terms of angular seperation per year, eg, "Annual Change" 1 mil or 3 minutes west. For maps and charts covering large areas where the magnetic declination changes from say, west to east the rate of change annual may be expressed as "Annual rate of Change 7 minutes increase/decrease for east/west declination". thus if the declination is east it increases at the rate of 7 minutes per year; if the declination is west it decreases at the rate of 7 minutes per year
How to Calculate the Grid Magnetic Angle.
Example:
Lets say the information we have from the map is as follows:
Grid magnetic angle, 02 deg 58 min W of grid north, July 2002. Annual change 14 min E.
Take the current year for this example lets say its 2003.
Subtract 2002 from 2003 = 1 year.
Remember the annual change was 14 min E.
Multiply this with the total year difference 14 min x 1 year = 14.
Subtract the 14 min from the grid magnetic angle of 02 deg 58 min = 02 deg 44 min.
Round this up to a whole figure, and the grid magnetic angle you would use for the current year would be 3 degrees.
If after doing your calculations and the grid magnetic angle was 02 deg 25 min, to round it up to a whole figure you would go back so it would be in this case 02 degrees.
Note: A good habit to get into is, every time you buy a new map, work out the grid magnetic angle, and mark it in pencil at the top of the map.
Remember: The above is only an example on how to calculate the grid magnetic angle. You would have to do it for the current year.
