Can I subtract a fixed offset between NGVD 29 and NAVD 88?

No single nationwide offset is safe. The relationship varies by latitude and local geoid behavior. Use published conversion tools or project documentation for the coordinates you care about.

Is "feet above sea level" the same as NAVD 88?

Colloquially people say "sea level," but inland pool guides usually cite a geodetic vertical datum. See MSL and NAVD 88 for how those ideas connect without being interchangeable labels.

Vertical datum for lake levels & gauges | Lake Insights Glossary | Glossary

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Vertical datum for lake levels & gauges | Lake Insights Glossary | Glossary | Lake Insights

Why it matters on the water

Anglers planning a ramp, marina operators watching clearance, and emergency managers tracking flood storage all depend on elevation labels that trace back to a datum. Swap datums without converting and you can misread how far the lake is above a rule curve or a hazard.

On Lake Insights, each lake's level chart and threshold lines are shown in the datum published for that project—often NGVD 29 on older Savannah River pools, NAVD 88 on many newer products, or project-specific pool zeros.

Technical context

Vertical datums anchor orthometric heights (along the plumb line relative to a modeled geoid). They differ from tidal datums used on coasts, and from arbitrary "pool top" zeros used only inside one reservoir's rulebook.

Label	Typical use
NGVD 29	Legacy USACE pool tables, many inland gauges
NAVD 88	Modern topo, benchmarks, newer federal products
Local pool zero	Operations target for one project—document per lake

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Why it matters on the water

Technical context

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Solunar Tables Explained

Depth Finder Lake Level Offset Explained

How Lake Levels Are Measured

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Why it matters on the water

Technical context

On real lakes