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.

 

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

 Information Required to Calculate the Grid Magnetic Angle (GMA)

On this example above it reads as this:

Grid Magnetic Angle is estimated to be 01' 58' (35 Mils) west of Grid North at the centre of the sheet in july 2009.

Annual rate of change is about 11' (3Mils) east

 Grid Magnetic Angle (GMA) Information on the Explorer 1:25,000 Scale Map

 

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.

 

Information Grid Magnetic Angle

At the time of writing, the magnetic angle is around 2 degrees West of Grid North and is decreasing by 10' per year. So in about 12 years time the magnetic angle will be zero, and subsequently East of Grid North.
So in 24 years it will be 2 degrees East, in theory at this level the grid magnetic angle could be confidently ignored for the next 25 years.