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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 within the New Orleans District. Benchmarks subside at very rapid rates in the coastal areas of Southeast Louisiana and at much slower rates near Alexandria and Deridder, LA.
The methodology used to shift historical survey data to NAVD88 (2004.65) will vary dependent upon many factors such as time, funds, accuracy requirements, etc. Generally there are four methods to determine the datum/epoch shift.

Field Measurements w/ Known Historical Elevation: This method will yield the most accurate values based on the historical reference marks. The reference marks will need to be recovered and occupied/surveyed using the guidelines in NGS Publication 58. The difference between the elevation used for the original survey and the elevation established from the new network will directly tie in the old work to the latest control. This will not account for any differential subsidence that occurred between the reference mark and the survey positions.

Field Measurements w/o Known Historical Elevation: When the reference benchmark is not recorded and unknown, some assumptions will be required such as what mark was used and what its elevation was. Again follow the procedures in NGS 58 to establish new elevations on the reference mark. The historical elevation will have to be assumed based on what was available at the time of design. The difference between the assumed historical elevation and the newly established elevation will be used to shift the survey to the new datum/epoch.

Common Published Marks in Survey Area: When time and money are constraints, the closest marks with published elevations in both datum/epochs can be used to determine an average shift for the area. This method contains many assumptions and therefore is the least accurate but may be of some use for projects that don't require accuracy.

CORPSCON: This method does not account for subsidence or the change in elevation from epoch to epoch. CORPSCON model was also tied to the published elevations at the time the conversion model was created which contained errors associated with the already deteriorating elevation accuracies. This method should not be used for anything other that a pure datum shift keeping in mind that subsidence is not accounted for.

The most accurate method to accomplish that is to use GPS to re-observe each and every benchmark used for an old survey of interest. There is absolutely no way to compute it; there are no computer programs that are reliable for such a conversion; old benchmarks must be re-occupied to perform a re-determination of the current elevation of the mark. Many parts of the United States are areas of relatively stable elevations. The entire State of Louisiana is an area of crustal motion – we subside different amounts in different places and at different times! In fact subsidence has been detected as far north as St. Louis. The speed we subside changes at the same spot, and the speed of subsidence differs from spot to spot. We are unable to predict crustal motion exactly, whether it’s in Louisiana or in Tokyo or in Southern California.
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 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. 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." 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 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.
While revolutionary in 1929, the old way of doing things has been improved with research in geodesy as well as the fact that some areas of the country’s topography have changed over time. NGVD29 was based on what is called a “constrained least-squares network adjustment.” Since that original system was devised, our knowledge of the Earth’s crust has increased such that we can now measure tiny incremental changes in our system of elevation benchmarks. New surveys have become increasingly difficult to “fit” into the old system without warping the new observations as they are forced to be compatible with the old values. A study by the National Academy of Sciences found that the old system of NGVD29 had deteriorated to become an incompatible reference system and that the North American Continent needed to start all over again with something better. The new North American Vertical Datum of 1988 is a great improvement over its predecessor in terms of mathematical techniques employed as well as being based on the Earth’s observed gravity field. NAVD88 is compatible in principle with the Geoid, the mathematical system of describing the relation of the Earth’s gravity field to elevations.

We must first define the two datums.

The National Geodetic Vertical Datum of 1929 (NGVD29), previously referred to as Sea Level Datum of 1929, was the network of over 20,000 miles of levels constrained to Mean Sea Level (MSL) at 26 tide stations around North America. The network was warped due to variations in the Local Mean Sea Level (LMSL) at those 26 tide stations. These variations introduced errors into the network adjustment. The datum is not mean sea level and was renamed the National Geodetic Vertical Datum of 1929 in 1973.

North American Vertical Datum of 1988 (NAVD88), completed in June of 1991 contained an additional 100,000 miles (15,000+ miles of new levels) of levels and was a minimally-constrained adjustment, constrained only to the primary tidal benchmark at Father Point/Rimouski, Quebec, Canada.

Approximately 100,000 miles of leveling had been added to the National Geodetic Reference System (NGRS) since NGVD 29 was created. In the early 1970s, NGS conducted an extensive inventory of the vertical control network. Many existing benchmarks were affected by crustal motion associated with earthquake activity, postglacial rebound (uplift), and subsidence. Other problems (distortions in the network) were caused by forcing the 100,000 miles of leveling to fit previously determined NGVD 29 height values. NAVD88 was created to eliminate those errors, incorporate the additional leveling, and to produce a new network that is consistent with both conventional and GPS leveling.
Because of crustal motion in Louisiana, such a determination is impossible in the 21st century. Subsidence due to crustal motion is a fact in the entire Lower Mississippi Valley, and research has shown that NAVD88 elevations as far north as Memphis, Tennessee are now in error by at least 0.20 foot since the new datum was published in 1990.

In Missouri, yes; but in Louisiana, absolutely NOT! The NAVD88 was developed as a replacement for the NGVD29 because the NGVD29 had become unreliable and out-of-date. The NAVD88 was published for most of the country by 1990, except for those regions of known crustal motion which included Louisiana. In 1992, the NAVD88 was published for Louisiana, but because of the continuing crustal motion of subsidence the lifespan of the NAVD88 benchmark elevations in Louisiana was limited to a few years.

CORPSCON uses the data files developed for VertCon by the National Geodetic Survey. The data files represent a snapshot in time for the instant when the NAVD88 was first published for Louisiana. Those data files are static; they do not change, and they only represent the differences in elevations at certain benchmarks in Louisiana in 1992. Using CorpsCon and/or VertCon to convert survey data in Louisiana will provide conversions that are at least 6 inches in error, an amount that is not acceptable for engineering and design applications. Conversion values determined with CorpsCon and/or VertCon should only be used for pure datum shifts, keeping in mind that subsidence is not accounted for. Additionally, any biases due to regional adjustments or realizations will contribute to the overall errors in the computed shifts. In areas of crustal motion, the computed values are based on the datum relationships at the time to model was created. So when using older NGVD29 epochs such as 1964/65, the differences between NGVD29 and NAVD88 are different than would be for the 1976 epoch.

The most accurate method to accomplish that is to re-observe each and every benchmark used for an old survey of interest. There is absolutely no way to compute it; there are no computer programs that are reliable for such a conversion; old benchmarks must be re-occupied to perform a re-determination of the current elevation of the mark. Many parts of the United States are areas of relatively stable elevations. Everywhere in the State of Louisiana is an area of crustal motion – we subside different amounts in different places and at different times! The speed we subside changes at the same spot, and the speed of subsidence differs from spot to spot. We are unable to predict crustal motion exactly, whether it’s in Louisiana or in Tokyo or in Southern California.

To obtain a current elevation at an arbitrary spot or at a benchmark in Louisiana, the procedure requires the services of a Registered Land Surveyor that uses a geodetic-quality dual-frequency survey-grade GPS receiver with a ground plane antenna & fixed-height tripod. That type of contraption is very different from the GPS receivers one can purchase at a department store or fishing & sporting goods emporium.

When the North American Datum of 1988 (NAVD88) was published for South Louisiana in 1992, some benchmarks that had NGVD29 values were included in the short list of benchmarks that had new NAVD88 elevations. For a few years, a computer program called VERTCON, sometimes used within another program called CORPSCON was used to convert those specific areas from one old datum to the new datum. That relation is no longer possible, since the surface of the Earth has changed so substantially since then due to subsidence. If that software is used for South Louisiana during the 21st century, the answers are guaranteed to be wrong.

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 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
http://www.ngs.noaa.gov/heightmod/Tech50.shtml
Updated Elevations for Coastal Louisiana
http://www.ngs.noaa.gov/heightmod/LouisianaControl.shtml

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.

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.

NGS is currently gearing up for another survey of South Louisiana which will determine NAVD88 elevations on approximately 300 benchmarks. This will be referred to as the 2006.?? Epoch.

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!
Actually, elevations of benchmarks have been changing for decades while we were still using the NGVD29 system. Each time Federal surveyors would come through Louisiana to re-observe benchmarks, a new epoch of benchmark elevations would become published by the National Geodetic Survey. With time, we discovered that the methodology was flawed because local benchmarks in the region were used as starting points, and their old elevations were used for the new surveys. Since those starting points had also subsided but were forced to remain having the same old elevations; each new survey provided different elevation values that were subsequently warped more and more.

With the new adjustment of the NAVD88, warps and the forcing of new surveys to fit old surveys are no longer done. The new elevations were compatible with each other when published in Louisiana in 1992, and although the original NAVD88 values are no longer valid (because of subsidence), the system of GPS CORS sites in Louisiana now allow elevation surveys to remain current as they are observed.
In metropolitan New Orleans, we sink approximately 3 feet per century or about 1 inch every 2 ½ years. Along the Louisiana coast, some areas are subsiding at the rate of 6 feet per century or 1 inch per year! We will continue to subside, but for different amounts in different places. Research indicates that the rates of subsidence are not constant, but appear to be episodic in some areas. That applies to both the surface-related differential subsidence that is due to the local soil conditions as well as to the deep-seated subsidence that appears due to consolidation of deltaic materials, the presence of geologic faults, and the bending of the lithosphere (the Earth’s crust) due to the sheer weight of the deltaic overburden.
Subsidence is a regional phenomenon which varies with respect to location. Each reach of levee was affected in varying amounts. Our Levee Section 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.
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.

Because we do not know the amount of error that existed in the benchmarks, we can not 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.
The Harris-Galveston Subsidence District (metro Houston) has severe subsidence problems that originate from copious groundwater pumping. The Corps of Engineers Reservoir at Addicks (West Houston) subsides at approximately 27mm/year! There are a series of ordinances that attempt to moderate the subsidence through rationing the amount of groundwater takeoff, and there has been some success in decreasing the subsidence rates. However, faults still account for some subsidence, and there seems to be less coordination of subsidence monitoring and publication of benchmark epochs in Houston than in the New Orleans District.

Additional Surveying Frequently Asked Questions at NGS FAQs.
CORS is the acronym for a GPS "Continuously Operating Reference Station". Originally established in the United States by the U.S. Coast Guard to provide a precise positional reference for shipboard navigators, the system has become popular with Land Surveyors, Engineers, and with Geologists that research the movements of the Earth’s continental plates.

Congress asked for a solution to improve the way reliable elevations and heights were being determined in the United States, and the National Geodetic Survey responded with a proposal for a Height Modernization Program to utilize GPS CORS sites as the basic elevation reference tool for the 21st century.

Around the same time, the U.S.S.R. fell with the Iron Curtain, and the U.S. Department of Defense began to de-classify the secret gravity model of the Earth that had been used for targeting Inter Continental Ballistic Missiles. That secret gravity model is now available to the general public, and it is one of the key components of the National Height Modernization Program. Since GPS does not provide elevations, the gravity model termed the Geoid is the connection between GPS heights and elevations. Useful for most areas of the world, the Geoid has been specifically refined and its accuracy enhanced for the United States. In fact, it is especially precise for the State of Louisiana because it represents the only way we can obtain current, up-to-date elevations! Using the CORS with the latest version of the Geoid is how Land Surveyors and Engineers are now obtaining reliable, current elevations for the rebuilding of the areas devastated by Katrina and Rita.

More information can be found at the NGS CORS Website.

The old way of doing things was based on "spirit leveling", which had its roots in the old surveys of the Babylonians, dating back some 3,000+ years. When the government establishes the elevation of a benchmark, that elevation is good as long as the benchmark does not move up or down. Louisiana is subsiding, and benchmarks become useless because they do not keep their elevations for more than a few years at best, some only for a year when located near the coast!

The CORS sites have GPS antennae bolted to buildings and structures in Louisiana. As the ground and the buildings subside, so does the antenna such that the reference elevation is computed and re-determined every few days - the value is always up-to-date!

A map of existing CORS in Louisiana can be found here.
"ac·cu·rate" 1. In exact conformity to fact: errorless.

Yes, GPS is more accurate than levels. The reason for that is because the accuracy criterion for First-Order Geodetic Leveling is an error of closure of (±3mm/km). To achieve accurate leveling results in the New Orleans District better than the capability of GPS, it would therefore be necessary to start at an accurate benchmark within 28 miles of the eastern border of Louisiana.

However, research has proven that the closest accurate (reliable) benchmark to Louisiana is east of Mobile Bay, Alabama!

Therefore, GPS is more accurate than levels in the New Orleans District, because of subsidence. However, there are no accurate benchmarks close enough to the New Orleans District to allow leveling results equal to or better than what is possible on a day-to-day basis with dual-frequency GPS receivers.
All GPS receivers provide ellipsoid heights; they do NOT provide elevations.

The difference between zero elevation on the North American Datum of 1988 (NAVD88) and zero ellipsoid height for South Louisiana is about twenty seven meters = 88.5 feet. Handheld GPS receivers (consumer grade), receive single frequency Coarse Acquisition code (CA code), where with Selective Availability turned off, has a nominal positional accuracy under good conditions of about ±15 feet in the horizontal.

Since the vertical accuracy of Handheld GPS receivers is about seven times worse than horizontal positions, 15 × 7 = 105 feet + 88.5 feet = ±194 feet vertical accuracy under good conditions!

That’s not bad for a $99 GPS receiver. For about $12,000 one can purchase a dual-frequency geodetic-quality GPS receiver that can provide ±2 centimeter accuracy over two days’ observations of at least 30 minutes each day, depending on baseline length. Of course, post-processing will require additional computer time to reduce the observed ellipsoid height to an elevation referenced to NAVD88. That is not possible with a handheld GPS receiver.
There are several reasons why this can occur:

1.) A particular “benchmark” was never part of the National Spatial Reference System (NSRS), to begin with. Perhaps another federal agency, even the Corps of Engineers, established the elevation of a benchmark but did not “bluebook” the observations and descriptions nor submit the data to the National Geodetic Survey for incorporation into the NSRS.

2.) A benchmark was originally published on the NGVD29, but was not included in the readjustment to the NAVD88 because it was not observed (re-leveled) in South Louisiana during the 1980s through the early 1990s.

3.) The published elevation has been deemed by the National Geodetic Survey to be unreliable and has been “pulled” from the published records in 2006.
The benchmark has been updated to a current elevation using GPS Leveling techniques. Since the elevation is reported to the closest 0.1 foot, then that published value is considered accurate to within one-half of that tenth of a foot, or ±0.05 foot. The benchmark has been updated to a GPS Leveling vertical accuracy criterion of ±2 centimeters. That corresponds to ±0.06 foot, which will round off to a value of 0.1 foot.
Mean Lower Low Water is a hydrographic datum and is dependent on locality. Since tidal regimes vary with river stages, weather, prevailing winds, currents, etc., the relation of MLLW to NAVD88 changes from place to place. MLLW is commonly associated with Mean Low Gulf (MLG).