OK, again, what is an Epoch?
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 the New Orleans District 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 New Orleans District. The only way to avoid creating new epochs in determining elevations in the New Orleans District is to use GPS Leveling techniques based
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 the New Orleans District 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 New Orleans District. The only way to avoid creating new epochs in determining elevations in the New Orleans District is to use GPS Leveling techniques based
