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AhaBulb: Stuff Everyone Should Know · ahabulb.com/scout-old-map-declination

Why might the declination printed on an old topo map be wrong today?

Magnetic north moves, so declination changes over time.

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  1. Paper maps shrink a little every year
  2. True north moves a few miles every year
  3. Magnetic north shifts, so declination changes over time ✔

Why: Declination changes over time and with location because Earth's magnetic field drifts. USGS warns that values on old maps may no longer be accurate, so check a current value.

Huh, didn't know that: USGS says declination values on current maps come from NOAA.

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My old map and I have something in common. We are both a little out of date.
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References

Answer: Magnetic north shifts, so declination changes over time. Declination changes over time and with location because Earth's magnetic field drifts. USGS warns that values on old maps may no longer be accurate, so check a current value.

  1. USGS FAQ: What do the different north arrows on a USGS topographic map mean? www.usgs.gov/faqs/what-do-different-north-arrows-a-usgs-topographic-map-mean
    Magnetic north (MN) shows the direction a magnetic compass would point at the time the map was published.
    Checked 2026-10-08.
  2. NOAA NCEI: Magnetic Declination www.ncei.noaa.gov/products/magnetic-declination
    Magnetic declination changes over time, and with location.
    Checked 2026-10-08.

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