Magnetic declination is indicated on USGS topographic maps. The map you are using for field mapping in a rural area of slow geologic change is dated 1954. The magnetic declination noted on this map is 16 degrees west of north. Can you use this map?

• A. Yes. Field check the magnetic declination at the site and adjust it prior to measuring strikes and dips
• B. No. You cannot use the map at all
• C. Yes, you can rely on the map as is
• D. Only if the map is newer than 1960

Answer: (A)

Magnetic declination is the angle between true north and magnetic north at a point. On that old map, the declination is 16 degrees west, so magnetic north lies 16 degrees west of true north. When you do field measurements with a compass, you’re reading relative to magnetic north, not true north. To translate those readings to a true-north reference, you apply the local declination. Because the value can drift over time (secular variation), an old map’s declination may not match today, but you can still use the map by determining the actual local declination in the field and applying that correction before recording strikes and dips.

So you can use the map, as long as you field-check the declination at your site and adjust your measurements accordingly. For example, with a west declination of 16°, a magnetic reading of a strike of 110° becomes a true strike of 126°.