Calculating the mean magnetic declination involves finding the average magnetic declination over a period, which is useful for understanding trends or for use in navigation adjustments.

Here’s a step-by-step guide to calculating it:

1. Gather Magnetic Declination Data for the Time Period

• Collect the magnetic declination values at regular intervals over the time period you’re interested in (e.g., yearly, monthly, or daily values).
• Make sure each value is in degrees (°) and, if necessary, adjust for east (positive) or west (negative) to standardise them.

2. Convert All Values to the Same Unit

• If your data includes minutes (‘) as well as degrees, convert everything to decimal degrees for simplicity:
  $Decimal Degrees=Degrees+\frac{60}{}$ 

  Decimal Degrees=Degrees+60Minutes​

3. Add Up All Declination Values

• Sum all magnetic declination values to get a total.

4. Divide by the Number of Values

• Count the number of data points (declination values) you collected.
• Divide the total sum by this number to get the mean magnetic declination.

5. Express the Mean Declination in Degrees and Minutes (if needed)

• If necessary, convert your answer back from decimal degrees to degrees and minutes by multiplying the decimal part by 60.

Example Calculation

Imagine you have yearly magnetic declination values for 5 years:

• Year 1: 10° 20′ E
• Year 2: 10° 30′ E
• Year 3: 10° 10′ E
• Year 4: 10° 25′ E
• Year 5: 10° 15′ E

Convert each to decimal degrees:

• Year 1:
  $10+\frac{20}{60}=10.33\mathrm{°}$ 

  10+6020​=10.33°

• Year 2:
  $10+\frac{30}{60}=10.50\mathrm{°}$ 

  10+6030​=10.50°

• Year 3:
  $10+\frac{10}{60}=10.17\mathrm{°}$ 

  10+6010​=10.17°

• Year 4:
  $10+\frac{25}{60}=10.42\mathrm{°}$ 

  10+6025​=10.42°

• Year 5:
  $10+\frac{15}{60}=10.25\mathrm{°}$ 

  10+6015​=10.25°

Add them together:

$10.33+10.50+10.17+10.42+10.25=51.67$

10.33+10.50+10.17+10.42+10.25=51.67

Divide by 5:

$\frac{51.67}{5}=10.33\mathrm{°} or 10\mathrm{°}20^{\prime } E$

551.67​=10.33° or 10°20′ E

The mean magnetic declination over the 5 years is 10° 20′ E.