NGVD29 vs NAVD88: which datum is your lake on?

An elevation only means something relative to a starting point, called a vertical datum. US lakes use two.

• NGVD29, the National Geodetic Vertical Datum of 1929, was the national standard for most of the 20th century. Most dams built before the 1990s, including many Army Corps and TVA reservoirs, still quote full pool, spillway and ramp elevations in NGVD29.
• NAVD88, the North American Vertical Datum of 1988, replaced it as the official standard. It is based on far more survey data and fixes distortions that had built up in the older system.

How big is the difference?

It depends on where you are. Across much of the East and the Gulf Coast the two datums are within about a foot of each other. Moving west, the gap grows to several feet in the Rocky Mountain states. For an exact conversion at a specific point, NOAA’s VERTCON tool gives the offset.

Why it matters to you

If a lake operator says a ramp is usable down to 1,060 ft NGVD29 and a gauge reports 1,062 ft NAVD88, you cannot compare the two numbers directly. Always check which datum each number uses.

On Lakeglance, each water level page states the datum when USGS publishes it: parameter 62614 is NGVD29 and 62615 is NAVD88. Gauges reporting gage height use a local reference instead of either datum, so their numbers are only useful for watching rises and falls.

A new national datum, based on satellite gravity measurements, is planned to replace NAVD88. When it arrives, lake elevations will shift again on paper even though the water does not move.