Convert elevations between NAVD88, NGVD29, and MLLW tidal datums for coastal and marine construction projects. Whether you're working on a pier, seawall, dredge project, or coastal utility, this tool gives a quick approximation of the datum difference by region. For precise work, always verify with NOAA's VDatum.
Tidal datums are vertical reference surfaces defined by tidal water levels. Unlike geodetic datums (NAVD88, NGVD29) which reference a specific benchmark, tidal datums vary from station to station because tidal range varies along the coast. A location in Northern California sees 5–6 ft tidal range; the Gulf Coast near Pensacola sees only 1–2 ft.
MLLW is the average of the lower low tide over a 19-year epoch. MHHW (Mean Higher High Water) is the upper reference for coastal flooding and regulatory purposes. The difference between MLLW and MHHW at a station is the tidal range.
For critical coastal construction (bridge clearances, FEMA flood maps, Army Corps permits), always use site-specific tidal benchmarks from the nearest NOAA tide gauge station or NOAA's VDatum tool.
NAVD88 (North American Vertical Datum of 1988) is the current standard vertical datum for the United States. It is referenced to a tidal benchmark at Father Point/Rimouski in Quebec. NAVD88 replaced NGVD29 and is the datum used for most current USGS topographic maps, FEMA flood maps, and construction projects.
MLLW (Mean Lower Low Water) is a tidal datum referenced to the average of lower low tide levels over the 19-year National Tidal Datum Epoch. MLLW is the standard datum for nautical charts, bridge vertical clearances, and coastal/marine construction. MLLW elevations are typically negative when expressed in NAVD88.
Coastal and marine projects reference MLLW because it ties to actual tide levels. A pier deck elevation of +8 ft MLLW means 8 feet above average low tide — directly meaningful for marine operations. Nautical charts, regulatory permits (Army Corps), and dredging projects all use MLLW or MHFW (Mean High Water) references.
This calculator uses representative regional averages. The actual datum difference varies significantly even within a region — by up to ±1 foot in some coastal areas. For precise conversions, use NOAA's VDatum tool (vdatum.noaa.gov) with your specific project coordinates. VDatum uses a nationwide grid of actual datum differences at 1-km resolution.
NGVD29 (National Geodetic Vertical Datum of 1929) was the previous national vertical datum. It's no longer actively maintained but appears on older construction records, legacy flood maps, and historical surveys. The difference between NGVD29 and NAVD88 ranges from about −1.5 ft to +3 ft depending on location.
Convert elevations between NAVD88, NGVD29, and MLLW tidal datums by coastal region. Approximate converter for coastal and marine construction projects. Use NOAA VDatum for precise conversions.
GPS receivers and total stations for coastal survey work.
Shop Express Tools →NAVD88 (North American Vertical Datum of 1988) is the current standard vertical datum for the United States. It is referenced to a tidal benchmark at Father Point/Rimouski in Quebec. NAVD88 replaced NGVD29 and is the datum used for most current USGS topographic maps, FEMA flood maps, and construction projects.
MLLW (Mean Lower Low Water) is a tidal datum referenced to the average of lower low tide levels over the 19-year National Tidal Datum Epoch. MLLW is the standard datum for nautical charts, bridge vertical clearances, and coastal/marine construction. MLLW elevations are typically negative when expressed in NAVD88.
Coastal and marine projects reference MLLW because it ties to actual tide levels. A pier deck elevation of +8 ft MLLW means 8 feet above average low tide — directly meaningful for marine operations. Nautical charts, regulatory permits (Army Corps), and dredging projects all use MLLW or MHFW (Mean High Water) references.
This calculator uses representative regional averages. The actual datum difference varies significantly even within a region — by up to ±1 foot in some coastal areas. For precise conversions, use NOAA's VDatum tool (vdatum.noaa.gov) with your specific project coordinates. VDatum uses a nationwide grid of actual datum differences at 1-km resolution.
NGVD29 (National Geodetic Vertical Datum of 1929) was the previous national vertical datum. It's no longer actively maintained but appears on older construction records, legacy flood maps, and historical surveys. The difference between NGVD29 and NAVD88 ranges from about −1.5 ft to +3 ft depending on location.