How to Calculate Magnetic Declination in Geography

How to Calculate Magnetic Declination in Geography Magnetic declination, also known as magnetic variation, is the angle between magnetic north (the direction a magnetic compass points) and true north (the direction towards the North Pole). It’s an important concept in geography and navigation.

To calculate magnetic declination, you can follow these general steps:

1. Get the Current Declination Information:
   • Check online resources or use declination charts provided by geological or mapping agencies. You can find this information for your specific location.
2. Identify Your Location:
   • Know the latitude and longitude of your location, as magnetic declination varies with geographic location.
3. Understand the Sign Convention:
   • Magnetic declination is expressed in degrees and minutes, and it can be east or west. If it is east, it is added to true north to get magnetic north, and if it is west, it is subtracted.
4. Adjust for the Year:
   • Magnetic declination changes over time, so use the most up-to-date information for the year you are interested in. Some charts provide an annual rate of change that you can use to estimate the declination for a specific year.
5. Calculate Magnetic Declination:
   • If the information is provided in terms of a yearly rate of change, you can use the following formula: $\text{Declination (for a specific year)}=\text{Base Declination}+(\text{Year}-\text{Base Year})\times \text{Rate of Change}$ Here, the base declination is the declination for a known base year, and the rate of change is the annual change in declination.
6. Adjust for East or West:
   • If the declination is east, add it to true north. If it is west, subtract it.
7. Apply the Magnetic Declination:
   • Adjust your compass reading by the calculated magnetic declination to convert from true north to magnetic north or vice versa.

Did you know? By convention, declination is positive when magnetic north is east of true north, and negative when it is to the west. Isogonic lines are lines on the Earth’s surface along which the declination has the same constant value, and lines along which the declination is zero are called agonic lines. The lowercase Greek letter δ (delta) is frequently used as the symbol for magnetic declination.

Video Guide on how to Calculate Magnetic Declination in Geography

How to calculate magnetic bearing

In order to calculate magnetic bearing, you follow the steps below:

• Step 1: Determine the true bearing E.g 75°
• Step 2: Determine the difference in years between the current year & the year
  the Magnetic declination was measured (on topographic map). E.g. 2015 – 2005 = 10 years
• Step 2: Calculate the total magnetic change
  E.g 10 years x 2’ West = 20’ West (On the topographic map the change is 2’ West every year)
• Step 3: Calculate the current magnetic declination
  E.g 16°3’ West + 0°20’ West, When the mean annual change is west you add, & if the change is East, you subtract. = 16°23’ West Therefore, the magnetic declination for 2015 is 16°23’ west from True North.
• Step 4: Calculate the magnetic bearing
  E.g True Bearing + Current Magnetic Declination
  = 75° + 16°23’
  = 91°21’ west.