Lakes & reservoirs

NGVD 29 vs NAVD 88: Why Lake Elevations Differ

By the Water Level Atlas team · Last reviewed Oct 21, 2026

A vertical datum is the zero point that elevations are measured from. In the US, most lake elevations are on one of two datums: the older NGVD 29 or the newer NAVD 88. The same water surface can have two different numbers, one on each datum, and the gap between them depends on where you are, so you should only compare numbers on the same datum.

What is a vertical datum?

When a lake is "at 650 ft", that means 650 ft above a reference surface. A datum is that reference. The Southwest Florida Water Management District puts it simply: "A datum is a standard position or level (i.e., starting point) from which measurements are taken."

Surveyors tie gauges to marked points called bench marks, whose heights are known on a national datum. Change the datum and every height changes, even though the water has not moved.

What is NGVD 29?

The National Geodetic Vertical Datum of 1929 was the national standard for decades. According to NOAA's National Geodetic Survey (NGS), it held mean sea level fixed at 26 tide gauges, 21 in the US and 5 in Canada. Despite the old habit of calling it "mean sea level", NGS says the datum "was not mean sea level, the geoid, or any other equipotential surface." It was named NGVD 29 in 1973.

Many reservoirs were designed and built when NGVD 29 was the standard. Their dam plans, pool levels and older records often still use it.

What is NAVD 88?

The North American Vertical Datum of 1988 replaced NGVD 29. NGS says it was established in 1991. NGS explains why the change was needed: since 1929, about 625,000 km of new leveling had been added, and forcing that new work to fit the old NGVD 29 heights had caused distortions of as much as 9 meters in places.

NAVD 88 is the current official US vertical datum. NGS plans to replace it as well with a new gravity-based datum, and says NAVD 88 itself is biased by about half a meter and tilted by about 1 meter coast to coast relative to modern geoid models.

How big is the difference between NGVD 29 and NAVD 88?

There is no single conversion number. The gap changes from place to place, and NGS publishes a model called VERTCON to estimate it for a given location in the lower 48 states. NGS says the model is accurate to about 2 cm (about 0.07 ft) and should not be relied on outside the lower 48.

Two real examples show the size:

Florida's example: Lake Panasoffkee's level of 39.24 ft on NGVD 29 is 38.36 ft on NAVD 88. As the district notes, the change affects only the number, not the actual depth of water.

Notice the direction. In the Florida example NAVD 88 numbers are lower; in the Minnesota example they are higher. Even within one USACE project, the Lac qui Parle manual gives four different corrections for four nearby sites.

Why does the same lake have two elevations?

Common reasons:

• The dam and pool levels were set on NGVD 29, but the gauge now reports on NAVD 88.
• One agency publishes on NGVD 29 and another on NAVD 88.
• Some projects use their own local project datum, set when the dam was built.

So "full pool is 660 ft" and "the lake is at 659 ft" might not mean the lake is 1 ft low. If the two numbers are on different datums, the real difference could be larger or smaller.

How should I compare numbers?

1. Look for the datum label (NGVD 29, NAVD 88, or "project datum").
2. Only subtract numbers that share the same datum.
3. If they differ, use the operator's published conversion for that site, not a number from somewhere else.

Using this on Water Level Atlas

On our lake pages, "vs full pool" appears only when the operator publishes a full-pool level on the same datum as the gauge reading. Where the datums differ, we leave the comparison out rather than apply our own conversion. Our methodology page explains this, and what is full pool covers the full-pool comparison.