Thanks for the reminder, W_D! Many of us who are older are still using older topo maps! The topographic features (other than roads and trails) may not change, and may even be less accurate on newer maps, so why buy new ones? The magnetic declination may have changed considerably. though.
Per this reference: The magnetic declination in a given area will change slowly over time, possibly as much as 2-25 degrees every hundred years or so, depending upon how far from the magnetic poles it is. Complex fluid motion in the outer core of the Earth (the molten metallic region that lies from 2800 to 5000 km below the Earth's surface) causes the magnetic field to change slowly with time. This change is known as secular variation. Because of secular variation, declination values shown on old topographic, marine and aeronautical charts need to be updated if they are to be used without large errors. Unfortunately, the annual change corrections given on most of these maps cannot be applied reliably if the maps are more than a few years old since the secular variation also changes with time in an unpredictable manner.
It's easy enough to search online before a trip for the current declination and pencil it in on our older maps if it has changed. That should be part of our pre-trip planning routine.
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