An epoch is a time stamp that is associated with a group of benchmarks that have been observed and adjusted on a particular date. That epoch also refers to any engineering or construction work designed and/or built based on those published benchmark values.
Since all of Louisiana is in a crustal motion zone, every benchmark has a limited lifespan because its elevation will be valid only for a limited amount of time. The same phenomenon affected old benchmarks published on the NGVD29 datum. Land Surveyors were aware that it was impossible to run differential levels from a benchmark published on one epoch to another benchmark published on a different epoch.
The same rule holds today, except that based on recent research (NOAA NGS Technical Report 50); the actual rates of subsidence are now recognized as they vary across the entire State of Louisiana. The only way to avoid creating new epochs in determining elevations in Louisiana is to use GPS Leveling techniques based on the regional system of GulfNet Continuously Operating Reference Stations (CORS). However, each point that has an elevation determined by GPS Leveling will itself have an epoch based on the date of that determination. That point will subside with time and will need re-determination based on local vertical subsidence velocities.
Ref: NOAA Technical Report 50: Rates of Vertical Displacement at Benchmarks in the Lower Mississippi Valley and the Northern Gulf Coast
Updated Elevations for Coastal Louisiana
NGVD29 Epochs in Southeast Louisiana: 1938 - adjustment based on SLD 1929, 1951 - adjusted forward in time to 1955, 1955 - tied to Morgan City & Mobile (‘29), 1963 - tied to Norco well (‘29 value) , 1968 - tied to ’63 lines, 1976 - tied to Index, AR. and Logtown, MS, 1984 - Orleans and Plaquemines Parishes tied to Waggaman & Rigolets ’76 values, 1986 - Jefferson and St. Bernard Parishes tied to ‘84 values. NAVD88 Epochs in Louisiana: 1992 - Original adjustment, 1994 - Observations in Orleans Parish, 2004.65 - Adjustment to GPS observations to validate NGS TR 50 subsidence rates, 2006.81 – results of post-Katrina Height Modernization project.