INTRODUCTION

Grid Magnetice Angle

Grid Magnetic Angle GMA

This information is found at the top centre and in the technical section of the map sheet and is required so the 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 about Grid Magnetic Angle

Angles Between North Points

The angles between these Norths are defined as follows:

Magnetic Declination: The angle between magnetic north and true north at any 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 or west from grid north.

Annual Magnetic Change

Annual Magnetic Change

The direction of magnetic north relative to true north, or grid north at a given point on the earth's surface changes continuously 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 change may be expressed as "Annual Rate of Change 7 minutes increase/decrease for east/west declinations". 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 GMA

Grid magnetic angle 02 deg 58 min West of grid North July 2006.

Annual change 14 min East.

Take the current year in this case 2007 subtract 2006 = 1 year.

Remember the annual change was 14 min East.

Multiply this with total year difference 14 min x 1 year = 14 min.

Subtract the 14 min from the GMA 02 deg 58 min = 02 deg 44min.

Round this up to a whole figure and the GMA you would use for the current year would be 3 deg

Note: If after calculating and your final answer was 02 deg 25 min you would round back to the whole figure i.e. 2 deg

Example 2

Grid Magnetic Angle of 05 deg 30 min W of Grid North 1988.

Annual change 09 min E.

Take the current year in this case 2003 suntract 1988 =15 years.

Remember annual change 09 min E.

Multiply this with the total year difference 09 min x 15 years = 135 min.

Divide the amount 135 min by 60 this will give you 2 deg 15 min

Suntract the 2 deg 15 min from the GMA 5 deg 30 min = 3 deg 15 min.

Round this up to a whole figure, and the GMA you would use for the year 2003 would be 3 deg.

You will notice that there was a big difference between the dates and the information on the map and the GMA.

              1988 - 05 deg 30 min

              2002 - 2 deg 58 min

However once the GMA was calculated for the current year which was 2003 for exercise purposes, and then rounded up to a whole figure, it came to exactly the same.

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