When converting coordinate data between different datums, obtaining a specific accuracy may require time-dependent processing when converting. This is because one datum may be defined relative to another and in a way that includes coefficients that are a function of time. Even in the case of a single datum, a measured position may have different coordinates when measured on two different dates, due to crustal motion. The latter situation is related to the former, since the reason for defining a time-dependent relationship between two datums is to minimize the effects of crustal motion in one datum.
Read the complete article: CueTip – Horizontal Time Dependent Positioning