Glossary
Datum
A datum is the agreed reference surface behind elevation numbers—most often a vertical datum for stage and pool heights. Comparing two readings only makes sense when both carry the same datum label, or a documented conversion between them.
Published May 16, 2026 · Editorial standards
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 |
On real lakes
Related lake intelligence
Pelican Lake (Orr)
/lakes/pelican-mn
Pelican Lake (Orr) catalogs elevation reference MSL—compare only to readings using the same labels.
Related lake intelligence
Lake Superior
/lakes/superior
Lake Superior catalogs datum IGLD85 and elevation reference MSL—compare only to readings using the same labels.
Related lake intelligence
Amistad Reservoir
/lakes/amistad
Amistad Reservoir catalogs datum NGVD29 and elevation reference MSL—compare only to readings using the same labels.
- 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.
Related terms: NAVD 88 · NGVD 29 · MSL · Elevation reference · Gauge
Reference information only. See data sources, editorial standards, or suggest a correction.