FAQ Detail

IS NGVD29 THE SAME AS MEAN SEA LEVEL?

No, it is not. Once thought to be close to Mean Sea Level (MSL), it was within a couple of feet for most of the northern rim of the Gulf of Mexico. The original datum was called the “Sea Level Datum of 1929” and it was the first vertical (elevation) datum established for an entire continent in the history of the world. With observations that started in the 19th century, a series of 26 tide gauges were recorded for over 19 years to establish Local Mean Sea Level (LMSL) for all of the coasts of the United States, Canada and Mexico. However, since LMSL varies from place to place because not only from astronomical phenomena but also due to local winds, river stages, storms, and local gravity ...

LMSL was not equal to "0.00 ft" everywhere.

"Zero" needed to be somewhere, so Galveston, Texas was selected as the "Primary Benchmark of the United States", and LMSL there was set equal to "0.00 ft" in 1929. That elevation of the mean sea in Galveston was within a couple of feet or so to what it was in Biloxi, Mississippi where the closest tide gauge was to New Orleans, back in the late 19th and early 20th centuries.

In an attempt to avoid confusion, the name of the "Sea Level Datum of 1929" was changed in 1972 to the "National Geodetic Vertical Datum of 1929", or NGVD29. No computations were performed, and no observations were made. This was a name change only.

Example Impact of Datum Variations on Constructed New Orleans Floodwall Elevations: Given the nearly universal presumption that NGVD29 and MSL were equivalent "sea level" datums, and that floodwall designs were computed relative to Lake Pontchartrain MSL, the actual constructed elevation on a typical floodwall in the London Avenue Outfall Canal is reduced by approximately 0.81':

Benchmark CHRYSLER RM 7.11 ft “NGVD” (82)

Benchmark CHRYSLER RM 6.30 ft LMSL (1960-1978 epoch) Difference: 0.81 ft In effect, floodwalls designed relative to a MSL or LMSL datum would have been constructed about 0.8 ft lower when using the NGVD29 geodetic datum from a 1982 adjustment as a reference.

Thus a floodwall designed to 14.0 ft NGVD (i.e., MSL) would actually be constructed to 13.2 ft relative to LMSL (1960-1972 epoch), or 13.1 ft relative to the 1983-2001 LMSL epoch.

Where are the levee tops now with subsidence?
Subsidence is a regional phenomenon which varies with respect to location. Each reach of levee was affected in varying amounts. The Levee Section of the New Orleans District Corps of Engineers should be able provide the differences for a given reach. In a nutshell, they are anywhere from 0.5’ to 2.0+' lower than the prior local vertical control indicated.

Haven't Corps projects sunk up to 2 feet giving protection heights of say, 10 ft instead of 12 ft?
Yes, some projects have elevation values that are 2’ lower. That does not mean that they sank 2’. We must make it clear that one can not simply subtract today’s NAVD88(2004.65) elevation from an NGVD29(64) elevation to get the amount of subsidence. Please see the explanation below.

The change in Datum is a change in where we measure from - to establish elevations on structures, benchmarks, etc. The datum shift from NGVD29 to NAVD88 is not constant. The datum surfaces are not parallel and therefore vary with the location. A datum change does not change the relationship of the levee heights to the water. Please see the explanation below.

The difference in the apparent elevations at our hurricane protection projects is a product of several factors.

1. Datum Shift: The zero reference (or where the ruler starts) was changed from NGVD29 to NAVD88. This has nothing to do with the physical elevation with respect to sea level. This is only a change in where the elevation is measured from. It's like if we measured the depth of your property from the edge of the street and got 145'. Then we change the datum, or where we measure from, and now we measure from the edge of the sidewalk to the back of your lot. We now get 137'. The change in the depth doesn't mean that your lot is smaller. It's just measured from a different starting point. It's the same concept.

2. Error in the Old Elevation: Due to regional subsidence, the previously published elevations were inaccurate. We don't really know the true elevation of the monument used for design and construction. This was caused when NGS performed local leveling and adjusted those measurements to marks that were assumed to be stable. Because the marks held fixed were in fact subsiding, the fixed elevations were inaccurate which caused all elevations in the local network to become obsolete. This amount of error is unknown.

3. Subsidence: Southern Louisiana is sinking due to many factors. This process causes our vertical control to become inaccurate as the elevations change, unless monitored. Until now, long level lines would have had to been performed every few years at the cost of $1500/mile from a stable region, such as Pensacola FL, to monitor the movement of the control network. GPS and Continuously Operating Reference Stations (CORS) can now be used to monitor the changes and is currently being used by the New Orleans District Corps of Engineers.

Because we do not know the amount of error that existed in the benchmarks, we cannot derive the amount of subsidence that has occurred between construction and today.

It's important to understand that the amount of change in elevation does not reflect the amount of settlement or subsidence.

The change in Datum is a change in where we measure from to establish elevations on structures, benchmarks, etc. The datum shift from NGVD29 to NAVD88 is not constant. The datum surfaces are not parallel and therefore vary with the location. A datum change does not change the relationship of the levee heights to the water. Please see the explanation below.