The Declination Diagram on Topographic Maps

The Early maps usually had an arrow indicating to the north direction. By showing the north direction, it is easy for the map user to identify other important directions like west, east and south.

However, in modern Topographic maps, the information regarding the direction of north is given in the form of a diagram known as THE DECLINATION DIAGRAM. This diagram is usually located in the lower margin of the map sheet.

The declination diagram shows the interrelationship of the true north, magnetic north and grid north for the given map sheet.

• True north (or geographic north) is the direction towards the geographic North Pole from any place on the earth’s surface. 
• Magnetic north is the direction towards the Earth’s North Magnetic Pole from any place on the earth’s surface. It is usually determined by a magnetic compass. 
• Grid north is the direction shown by the vertical grid lines on maps drawn with a grid reference system such as UTM or MGRS. 

The declination diagram includes the following;

1. Prongs or short lines showing the direction of the true north, magnetic north and grid north, as measured from the center of the map sheet, in the survey year indicated.

• The line showing the direction of true north is usually marked with a 5-point star on its top-end, and labelled with letter TN.  
• The line showing the direction of magnetic north is usually marked with half a half arrowhead, and labelled with letters MN.  A left half-arrowhead is used when MN lies on the left of GN and a right half-arrowhead is used when MN lies on the right of GN.
• The line showing the direction of grid north is usually bare on its top-end, and labelled with letters GN.

2. The angular relationship of the three north references, as measured from the center of the map sheet, in the survey year indicated. 

There are three angles relating to the declination diagram. They include;

• The Magnetic declination (or magnetic variation)

This is the angle between magnetic north and true north. A map user is likely to use a magnetic compass, which usually points towards the magnetic north rather than true north, to determine direction in the field. Unfortunately, for most maps, the coordinate systems are based on true north. Therefore, knowing the magnetic declination can help a map user to easily determine the direction of the true north for a given map sheet.

• Grid convergence

This is the angle between grid north and true north. On maps drawn with UTM or grid reference system, it is important to know the difference between GN and TN.  By knowing the grid convergence, a map user can easily determine the direction of the true north for a given map sheet.

• Grid declination (or Grid-magnetic angle  or grid variation)

This is the angle between grid north and magnetic north. For maps drawn with UTM or grid reference system, it is important to know the difference between GN and MN, especially if the map user is using a magnetic compass to determine direction in the field.

3. Information on the annual rate of change of the magnetic poles.
A magnetic compass usually points towards the north magnetic pole of the earth. However, Due to constant movement of molten iron in the core, the position of both north and south magnetic poles is also constantly changing.  This means that the direction of MN keeps on changing slowly overtime. The rate and direction at which it changes annually is usually specified right under the declination diagram.
This information on annually rate of change will be useful to map user, particularly if the map is several years old. The changes in direction of the magnetic north will eventually cause changes in the magnetic declination and grid declination.