Lake Elevation & Volume Converter

Understanding NGVD29 vs. NAVD88 Vertical Datums

When measuring water elevations, the number only has meaning if you know what "zero" represents. In the United States, water monitoring agencies and surveyors rely on two primary vertical reference datums:

• NGVD29 (National Geodetic Vertical Datum of 1929): The legacy historical datum used when many major dams were engineered and constructed by the U.S. Army Corps of Engineers and the Bureau of Reclamation in the 1930s through 1970s. It was based on tide gauge averages along North American coastlines.
• NAVD88 (North American Vertical Datum of 1988): The modern, technologically refined geodetic datum established using satellite observations and extensive leveling networks across the continent.

Why does this difference matter? In many parts of the United States, NAVD88 is between 0.5 and 2.0 feet higher or lower than NGVD29 at the exact same physical spot. If a lake authority lists full pool as 681.0 feet NGVD29, but a modern property survey marks your dock at 682.0 feet NAVD88, you cannot simply subtract the two numbers without adjusting for the local datum shift!

Learn more in our complete NGVD29 vs NAVD88 Glossary Guide →

What Is an Acre-Foot of Water?

An acre-foot is the volume of water required to cover exactly one acre of flat land (43,560 square feet) to a depth of one foot. One acre-foot equals exactly 325,851.4 U.S. gallons.

In water management, hydrologists and water utility directors prefer acre-feet because reporting municipal reservoirs in gallons yields impossibly large numbers with dozens of zeros. As a helpful rule of thumb in American hydrology, one acre-foot is roughly the amount of water consumed by two to three average suburban households in a full year.