The National Datum Epoch lasts how many years?

• A. 19 years
• B. 10 years
• C. 25 years
• D. 5 years

Answer: (A)

The concept being tested is how a datum uses a fixed reference time, called an epoch, for its coordinates. Coordinates tied to a national datum aren’t static forever—the Earth’s crust moves, sea levels change, and surveying methods improve. To keep navigation and mapping accurate, the national datum is refreshed on a set schedule, updating the reference points and recalibrating the grid.

The 19-year length is the chosen interval for the National Datum Epoch. It’s a practical cadence that balances keeping the datum current with not requiring constant redefinition. When the epoch ends, a new epoch starts, the datum is updated to reflect the latest measurements, and charts, maps, and navigation computations that rely on that datum are aligned to the new reference.

In short, a datum epoch provides a moving reference frame, and 19 years is the standard period used for the National Datum.