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Ellipsoidal to Orthometric Height Converter

Convert ellipsoidal (GPS) height to orthometric height using the geoid separation N: H = h − N.

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How to use this calculator

Inputs

H = h − N (and h = H + N) with a signed geoid–ellipsoid separation N that you supply from a stated model.

Signed: negative where the geoid lies below the ellipsoid (most of Australia). Required — a blank is not 0.

More options

Enter the values and press Calculate.

Or load an example to see a worked result.

Your heights, signed
Not to scale
Your heights, signedSide view with the ellipsoid as a straight line, the geoid as a wavy line and the point above both, with the signed heights h, N and H dimensioned.EllipsoidGeoidPointh N < 0H H = h − N (signed)
H = h − N. Where the geoid is below the ellipsoid N is negative, so H is larger than h. Example: h = 45.2 m, N = −30.5 m → H = 75.7 m.

Batch from CSV

One row per calculation, with a header row. Values use the units currently selected in the form above. Up to 10,000 rows; nothing is uploaded.

How the method works

A fixed worked example to explain the method — it is not your result.

Signed separation must match the stated datum/model
Illustrative method
Signed separation must match the stated datum/modelSide view with the ellipsoid as a straight line, the geoid as a wavy line and the point above both, with the signed heights h, N and H dimensioned.EllipsoidGeoidPointh N < 0H H = h − N (signed)
H = h − N. Where the geoid is below the ellipsoid N is negative, so H is larger than h. Example: h = 45.2 m, N = −30.5 m → H = 75.7 m.

Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.

How to use this calculator

  1. Choose h → H (GNSS height to orthometric) or H → h.
  2. Enter the height and the signed separation N — negative where the geoid lies below the ellipsoid, as across most of Australia. N is required; a blank is never treated as 0.
  3. Say where N came from and which horizontal datum the position uses, so mismatched pairs can be flagged.
  4. Optionally enter uncertainties for both the height and N to combine them.

How it works

H = h − N, and in reverse h = H + N.

N must come from a model that matches the position's datum: AUSGeoid2020 with GDA2020, AUSGeoid09 with GDA94. AUSGeoid models give AHD heights, which include an offset from the pure geoid.

No global constant N is assumed, and nothing is looked up online — you supply N.

With uncertainties for both, σ = √(σh² + σN²), which assumes the two errors are independent.

Worked example

h = 45.2 m, N = −30.5 m. H = 45.2 − (−30.5) = 75.7 m. Reversing 75.7 m with the same N returns 45.2 m.

Common mistakes

  • Getting the sign of N wrong: subtracting a negative N raises the height.
  • Using an AUSGeoid09 value with GDA2020 coordinates (or the reverse).
  • Calling the result 'sea level' without saying which vertical datum it is on.

Frequently asked questions

Where do I get the geoid separation N?

From a geoid model for your datum (e.g. AUSGeoid2020 for GDA2020, GEOID18 in the US). GNSS gives ellipsoidal height; subtract N to get the orthometric height.

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Cite this tool

Zerdly. (2026). Ellipsoidal to Orthometric Height Converter. Retrieved from https://www.zerdly.com/tools/ellipsoidal-height-to-orthometric-height-converter/

Tip: Enter any known values to calculate the remaining results.

Designed to run locally in your browser. Tool inputs are not intentionally stored by Zerdly unless stated otherwise.