If a field map is dated 1954 and shows a magnetic declination of 16 degrees west of north, can you use it for field work?

• A. No, it cannot be used at all
• B. Yes, field-check the declination at the site and adjust before measuring strikes and dips
• C. Yes, you can rely on the map without changes
• D. Only if you also have a current satellite image

Answer: (B)

Magnetic declination changes over time, so a map from 1954 shows the declination for that date, not today. You can still use the map, but you must verify the current local declination at the field site and apply a correction to your readings before measuring strike and dip. In this case, you’d determine today’s declination (west of north by the latest value, whatever it is) and adjust your compass readings accordingly to convert magnetic bearings to true bearings. If the declination at the site is still west by 16° today, you’d subtract 16° from magnetic readings to get true bearings (and document the correction used).

This approach keeps the map as a reference while ensuring measurements reflect current orientation. Relying on the old declination without checking would lead to systematic errors in strike/dip measurements, and having a current satellite image doesn’t negate the need to correct for declination.