Before a field investigation in a region of complex geology, the most reliable way to determine magnetic declination at your location is to:

• A. Estimate from a standard map
• B. Use a simple compass and guess
• C. Contact NOAA for the magnetic declination for your latitude
• D. Use a local rock magnetism sample

Answer: (C)

Magnetic declination is the angle between true north and magnetic north, and it changes with location and time. In areas with complex geology, local magnetic anomalies can distort readings, so you need a precise, site-specific value rather than a rough guess.

The most reliable source is an official magnetic field model that provides current declination for your exact coordinates and date of fieldwork. NOAA maintains the World Magnetic Model and publishes declination values keyed to latitude and longitude (and time), so looking up your location’s current declination gives you the accurate correction you need to convert compass readings to true bearings.

Using a simple compass and guessing or estimating from a map can lead to significant errors, especially in regions with complicated geology where local anomalies matter. A local rock magnetism sample would tell you about the rock’s past magnetization, not the present-day direction of Earth's magnetic field at your location, so it isn’t appropriate for determining current declination.