What is the best method to obtain local magnetic declination when using an old map?

• A. By orienting the map along a straight road and measuring the orientation with a compass
• B. By using satellite magnetic data
• C. By measuring the magnetic field at the map's center with a magnetometer
• D. By counting the number of compass readings

Answer: (B)

To orient an old map, you need the local magnetic declination for the map’s location and the date it represents. Satellite magnetic data provide a global view of Earth’s magnetic field and, using models like the World Magnetic Model or IGRF, you can compute the declination at any location for a given date. This approach accounts for how the field changes over time and gives a consistent value for the map’s area, without being skewed by nearby metal objects or road directions. Other methods rely on local observations or instruments that can be distorted by ferrous materials, road construction, or calibration issues, making them less reliable for establishing true declination. Use satellite data to obtain the declination for the map’s location and epoch.