In the context of elevations, a Datum is a reference system that is used to compare elevations of various places to a certain height. Commonly associated with the concept of local mean sea level, the current system of elevations in North American is called the North American Vertical Datum of 1988. Although it is not the same as local mean sea level, it is usually within a foot or so of that as it varies from place-to-place because of tidal variations, winds, currents, and river stages in South Louisiana. A modern Datum does not change with time, and it stays the same. However, elevations change in Louisiana with time because of subsidence. Since the Datum does not change and elevations do change with subsidence, then the values of benchmark elevations do in fact change with time.
An Epoch is a collection of elevation benchmarks that have been determined for a given instant in time. Although the benchmarks continuously change with time because of the inexorable subsidence, benchmarks of a certain epoch are used by surveyors and engineers for a short period of time before their elevation values have changed beyond standard engineering specifications. How long an epoch is used depends on where the benchmarks are located in Louisiana. Benchmarks subside at very rapid rates in the coastal areas of Southeast Louisiana and at much slower rates near Alexandria and Deridder, LA.