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When discussing magnetic declination, the year of the measurement is crucial because magnetic declination varies over time due to changes in the Earth's magnetic field. This means that the angle between magnetic north and true north can differ significantly from one year to the next. Accurate surveys must account for the specific year in which the declination was measured to ensure precision in navigation and mapping.

While other factors, such as the location of the survey, can influence the declination, it is the temporal aspect—reflected by the year—that ensures the measurement is relevant and accurate. For example, a survey conducted in a different year may lead to inaccuracies if the transient nature of magnetic declination is not accounted for.

In contrast, the type of instrument and the method of calculation can affect how declination is measured, but they do not capture the inherent variability of declination over time.