Description
Subtract supplied geoid undulation from ellipsoidal height to obtain orthometric height.
Geographic coordinates use explicit angular, axis and vertical-datum conventions. Unwrapped longitude difference differs from the shortest circular separation, and a north-based bearing differs from a rightward-zero screen rotation. Ellipsoidal and orthometric heights require a compatible geoid-undulation model.
Subtract supplied geoid undulation from ellipsoidal height to obtain orthometric height.
When to use this tool
Use Ellipsoid Height Geoid Offset Pair when checking the stated geographic label, angular or height-reference convention.
How to use the tool and how it works
- Enter ellipsoidHeight, geoidUndulation. Use the units and conventions below.
- Run the tool. Correct any format, range, or cross-field input error before interpreting the output.
- Read orthometricHeight. Use the output convention below.
Return ellipsoidHeight-geoidUndulation as orthometricHeight on one compatible length and datum basis.1
With compatible reference systems, orthometric height H equals ellipsoidal height h minus geoid height N. This arithmetic does not select or evaluate a geoid model.2
| Input field | Meaning and units | Accepted input |
|---|---|---|
| ellipsoidHeight | Ellipsoid Height | Required. Finite number. |
| geoidUndulation | Supplied ellipsoid-minus-orthometric separation on compatible datums and units. | Required. Finite number. |
| Output field | Interpretation | Returned shape |
|---|---|---|
| orthometricHeight | Orthometric Height | Required. Finite number. |
{
"input": {
"ellipsoidHeight": 120,
"geoidUndulation": 30
},
"output": {
"orthometricHeight": 90
}
}Limitations and assumptions
- Geoid undulation is supplied; no geoid model, location, tide convention or actual vertical-datum compatibility is determined.
- Inputs must satisfy both the field types and the cross-field conditions above. Invalid input causes an error; a successful false flag or documented placeholder is a different result.
- Numeric calculations use binary floating point unless exact decimal-integer arithmetic is explicitly stated. No hidden measurement tolerance or decimal rounding is added; nonfinite or out-of-contract results are rejected. Numeric signed-zero results are exported as zero.
Alternative or Complementary methods
Use authoritative coordinate/datum definitions and appropriate geodesic, projection and geoid models for actual locations, distances and height conversion.
References
-
Calculation and input contract: geography/ellipsoid-height-geoid-offset-pair — tool.io
-
Height relationships in converting GPS height to orthometric elevation — NOAA National Geodetic Survey
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