To calculate magnetic declination in geography, you need to follow these steps. Magnetic declination is the angle between true north (geographic north) and magnetic north (the direction your compass points), and it changes depending on your location and the year.

Steps to Calculate Magnetic Declination

1. Find the Declination Value on a Map
   • Many topographic and navigation maps include a declination diagram or note showing the magnetic declination for the area. This value is usually displayed in degrees and is marked as east (E) or west (W) of true north.
2. Adjust for Annual Changes in Declination
   • Magnetic declination changes over time, so maps often include an annual change rate in minutes (e.g., “declination decreases 2′ west per year”).
   • Calculate the difference in years between the map’s publication date and the current year.
   • Multiply the annual change rate by the number of years to find the total change in declination.
3. Add or Subtract the Annual Change
   • If the declination change is eastward, add it to the original declination value.
   • If the change is westward, subtract it from the original value.

   For example, if the original declination is 10°E with an annual change of 2′ west and the map is 5 years old, the updated declination would be:

   $10^{\circ }-(2^{\prime }\times 5)=10^{\circ }-10^{\prime }=9^{\circ }50^{\prime } E$10∘−(2′×5)=10∘−10′=9∘50′ E

4. Use the Updated Declination
   • With the updated magnetic declination, you can now accurately adjust compass readings by accounting for the angle between true north and magnetic north.

Example

If the map indicates a declination of 7°W with an annual change of 1′ east and the map is 10 years old, you would calculate:

$7^{\circ }-(1^{\prime }\times 10)=7^{\circ }-10^{\prime }=6^{\circ }50^{\prime } W$

7∘−(1′×10)=7∘−10′=6∘50′ W

This updated declination value can then be used to correct compass readings for your specific location.