NOAA NGS TR 50 predicted subsidence rates of several thousand First-Order benchmarks in Louisiana, Mississippi, and Alabama. During 2004, geodetic survey crews, under the direction of the NGS observed elevations of 99 selected benchmarks in South Louisiana to verify the predicted subsidence rates listed in NOAA NGS TR 50. That collection of 99 rapidly-subsiding benchmarks has now been the only benchmarks remaining to be published for South Louisiana, and their subsidence rates have been validated by direct observation with GPS Leveling techniques. Those 99 benchmarks are the only benchmarks that are expected to be now maintained by the National Geodetic Survey and the Louisiana Spatial Reference Center.
The published epoch of those 99 benchmarks is termed "2004.65". The "2004.65" indicates that the mid-point of the field observations was on the 238th day of 2004 (65/100*366 = Day 238, 2004 was a leap year), or August 25th, 2004.
A more recent epoch is termed “2006.81,” and is a larger group of 300 benchmarks in Louisiana.
Why do we keep changing the benchmark elevations? The elevations of all benchmarks in the New Orleans District are constantly changing as we subside or sink. They change at varying rates from perhaps less than 10 mm/year to over 27 mm/yr. That corresponds from 1 inch every 2½ years to 1 inch every year, and at times even more than that! Since we are concerned about where we are with respect to the water around us, it only takes a few years for the accumulation of subsidence rates to sizeable amounts that adversely affect our engineering designs and our constructed works. We don’t change the elevations, the elevations change on us!