The relation varies from place to place and from time to time, because Mean Low Gulf is a Hydrographic Tidal Datum that is based on astronomical tides, prevailing winds, prevailing currents, and the variation in flow of the Mississippi River. NAVD88 is a Geodetic Datum that is based on the current gravity field of the Earth (as realized in this context), in the Louisiana littoral zone. Both datums vary in time and in place, both collectively and individually. As of June 2006, the only locations where the conversion between MLG and NAVD88 have been preliminarily determined by NOAA CO-OPS have been at : The Rigolets: 0.118 ft. 0.78 ft = -0.662 ft NAVD88 New Canal (17th Street Canal): 0.252 ft 0.78 ft = 0.528 ft NAVD88 East Bank, Bayou Labranche: 0.407 ft 0.78 ft = 0.373 ft NAVD88 The 0.78′ was the difference between the Mean Sea Level and the Mean Low Water at the gauge in Biloxi, Ms Note: All points being on the south shore of Lake Pontchartrain.