The 'epoch date' refers to the date associated with the coordinates of a control station. An epoch date is a necessary part of a complete datum or reference frame name, because coordinates can change with time.

• A. It is optional
• B. It is the date the data was collected
• C. It is the date associated with the coordinates of a control station; coordinates can change with time
• D. It is the date of the last update to the datum

Answer: (C)

In surveying, coordinates are tied to a reference frame that can change over time, so the time context of those coordinates must be specified. The epoch date is exactly that time stamp attached to the coordinates of a control station, indicating when those coordinates were valid. Since positions can drift due to factors like tectonic motion and updates to the reference frame, knowing the epoch lets you interpret the numbers correctly for the right moment. It’s not simply the date the data were collected, nor just the last update to the datum; the epoch is about the time at which the coordinates themselves are valid within the datum.