MLG to MLLW Vertical Datum Conversion

Abstract: The U.S. Army Corps of Engineers, New Orleans District is converting the vertical datum for all coastal navigation projects from Mean Low Gulf (MLG) to Mean Lower Low Water (MLLW) in accordance with USACE (2014), memorandum directing conversion from USACE HQ. This memorandum describes new policy for federal navigation projects where the decision documents supporting project authorization and the project authorization in law do not reference the Mean Lower Low Water (MLLW) datum.

This report documents the calibration / relationship between the localized MLG as used in practice for this project and the NOAA-maintained MLLW that provides for no impact in current project channel maintenance practices. The report provides details on how this relationship was determined and will be applied.

The North American Vertical Datum of 1988 (NAVD88) is a geodetic datum that is defined and maintained by the National Geodetic Survey (NGS). This datum is typically used for surveying (in addition to design and construction) and can be related to other datums as needed, to ensure project datums are referenced as required.

Mean Lower Low Water (MLLW) is a tidal datum that is defined and maintained by the National
Oceanic and Atmospheric Administration (NOAA). This tidal datum is defined as the average of the lowest of the two daily low water heights observed over the National Tidal Datum Epoch (which spans a 19 year period).

Mean Low Gulf (MLG) is a legacy terrestrial datum that was originally defined relative to local
mean sea level as observed at the Biloxi gage in 1899 in the Gulf of Mexico. It has been used as a navigation (and construction) reference datum in coastal waterways such as the Gulf Intracoastal Waterway (GIWW) and the coastal portion of the Mississippi River navigation channel (Reference 2). MLG was intended to represent the low water level of the Gulf of Mexico, and was defined by District memorandum in 1944 as being 0.78 feet below local mean sea level as it was understood at that time. The origin of the 0.78 foot offset between MLG and LMSL is not known precisely; however, this value is half the tidal range at the Biloxi (Cadet Point) Tidegage (NOAA gage 8743735). Therefore, MLG as defined is equal to Mean Low Water (MLW) at Biloxi. Mean Low Water is the average of all low tides, whereas Mean Lower Low Water is the average of only the lower of the two daily low tides. The Gulf of Mexico has diurnal tides (one low tide per day), so the difference between MLW and MLLW is academic. At Biloxi, the two are approximately one tenth  of one foot apart, which is beyond the precision of either dredging or hydrographic
surveying. Consequently, it seems very likely that MLG was intended to represent the average low tide condition in the Gulf of Mexico, so that a given draft in MLG would be, on average, navigable during low tide. Mean Lower Low Water is presently 0.46 ft. below local mean sea level at Pilottown, LA, and 0.6 ft. below local mean sea level at Pilot’s Station East (at the mouth of Southwest Pass).2

Therefore, in theory, MLG and MLLW are essentially equal as they are within 0.2 - 0.4 ft, as related to MSL. The intent and application of MLLW and MLG were and are, also in theory, defined to represent the same water condition; as a tidal datum of a lowest daily water level that will be typically observed for that location. 

LIST OF ACRONYMS
- AHP: Above Head of Passes
- BHP: Below Head of Passes
- CEPD: Comprehensive Evaluation of Project Datums
- EDR: Engineering Documentation Report
- HQ: Headquarters office of the United States Army Corps of Engineers
- IPET: Interagency Performance Evaluation Task Force
- LCA: Local Cooperation Agreement
- LWRP: Low Water Reference Plane
- MLG: Mean Low Gulf datum, as a historic reference
- MLGSWP: In practice usage of Mean Low Gulf as localized for this project
- MLLW: Mean Lower Low Water datum
- MVN: Mississippi Valley New Orleans District
- NAVD88: North American Vertical Datum of 1988
- NOAA: National Oceanic and Atmospheric Administration
- NWLON: National Water Level Observation Network
- OPUS: Online Positioning User Service
- SWP: Mississippi River Southwest Pass
- USACE: United States Army Corps of Engineers
- USACE MVN: United States Army Corps of Engineers New Orleans District

Citation: US Army Corps of Engineers New Orleans District (2016) MLG to MLLW Vertical Datum
Conversion. Engineering Documentation Report EDR-OD-01, Prepared by: US Army Corps of Engineers New Orleans District Engineering
Division, 02 November 2016.

URL: http://www.mvn.usace.army.mil/Portals/56/docs/Navigation/EDRs/EDR-%20SWP-%20EDR-OD-01__2016-11-02.pdf