What does the term "datum shift" refer to?

The term "datum shift" specifically refers to the discrepancies that arise from different geodetic datums. A geodetic datum is a reference system that defines how the position of points on the Earth's surface is measured and represented. Different datums can have varying definitions of the Earth's shape, size, and gravitational field, leading to variations in coordinate systems. Therefore, a datum shift occurs when there is a need to convert or compare data from one datum to another, resulting in differences in coordinates that must be reconciled for accurate mapping and surveying.

In contrast, the other options don't capture the essence of what a datum shift entails. Equipment wear results in measurement changes but isn't related to the concept of geodetic references. Collecting data over different time periods would involve temporal changes rather than shifts between reference systems. Additionally, while terrain can influence GPS readings, this phenomenon does not pertain to the foundational shifts between different geodetic datums. Thus, the correct choice highlights the critical nature of understanding how datums can differ in geodesy.