Vertical datum / vertical reference (practical guide)
This page explains why “elevation” is not always directly comparable between services. It’s written for field use + app development. If you’re producing engineering deliverables, you must rely on authoritative project control and documented metadata.
TL;DR
Orthometric heights (often NAVD88 in the U.S.) are what most people think of as “elevation.”
Ellipsoidal heights are different and require a geoid model to convert.
If a provider doesn’t state the vertical reference, treat the number as best-available and verify.
What you’re measuring
Orthometric height
Height above an approximation of mean sea level (gravity-based). In the U.S., the common engineering datum is
NAVD88. Orthometric heights depend on a geoid model.
Ellipsoidal height
Height above a mathematical ellipsoid (WGS84/ITRF). GNSS receivers often output ellipsoidal height.
Ellipsoidal heights can differ from NAVD88 by many meters.
Geoid undulation (N)
The offset between the ellipsoid and geoid at a location. A common relationship is:
H ≈ h − N
where H is orthometric height, h is ellipsoidal height.
Why this matters for apps
- Comparing sources: two “elevations” can differ because they use different datums, not because one is “wrong.”
- Flood/SLR work: small vertical differences (0.1–0.5 m) can materially change outcomes in low relief areas.
- GNSS field notes: your phone/GNSS device may report ellipsoidal height unless configured otherwise.
Common pitfalls
- Units mismatch: meters vs feet. Always check units and conversions.
- DTM vs DSM: bare earth vs “top of canopy/buildings.”
- Resolution mismatch: 1m LiDAR vs 90m SRTM will not agree in complex terrain.
- Datum not declared: many services omit explicit vertical reference in responses.
Recommended workflow for “best available” elevation
- Use the Elevation Explorer same-origin API result as the baseline (it carries provenance + validation).
- Enable one or two cross-check sources when needed (not by default) to reduce rate-limit risk.
- If sources disagree: sample nearby points, inspect resolution, and confirm vertical reference for your use case.
- For engineering: confirm against authoritative control/benchmarks for the project.