Ellipsoidal vs Orthometric Height
Ellipsoidal height is measured from a smooth mathematical model of the earth — an ellipsoid — while orthometric height is measured from the geoid, the surface gravity actually defines; the two can differ by tens of metres at the same point. Horizontal coordinate systems get all the attention, but the mistake that quietly ruins elevation work is vertical: a receiver hands you one kind of height, the map beside you uses another, and nothing in either file says so. This guide explains why they differ and the one habit that keeps you out of trouble.
Two different reference surfaces
Ellipsoidal height is measured from a smooth mathematical model of the earth — an ellipsoid. It is a clean geometric surface with no relation to gravity, which is exactly what satellite positioning needs.
Orthometric height is measured from the geoid: the surface that gravity actually defines, roughly mean sea level extended under the continents. It is lumpy, because the earth's mass is not evenly distributed.
Both are "height", both are in metres, and they can differ by tens of metres at the same point.
| Property | Ellipsoidal height | Orthometric height |
|---|---|---|
| Measured from | The ellipsoid — a smooth mathematical model of the Earth | The geoid — the gravity-defined surface near mean sea level |
| Produced by | GNSS receivers and drone surveys with GNSS control | Levelling to benchmarks; national vertical datums |
| Follows gravity? | No — water does not flow downhill along it | Yes — what drainage, grading and flood levels need |
| How they relate | Orthometric ≈ ellipsoidal − N (geoid separation) | N spans roughly −100 to +85 m globally and varies across a country |
The geoid separation
The gap between the two surfaces is called geoid separation (often written N). The relationship is simple: orthometric height ≈ ellipsoidal height − N.
N is not small and not constant. It ranges from roughly −100 to +85 metres globally, and it varies across a country. This is why you cannot fix the problem with a single offset unless your site is small.
Why it matters in practice
Water does not flow downhill along the ellipsoid. Drainage, grading, flood levels and anything else driven by gravity must be computed on orthometric heights, or the design is simply wrong.
Volumes are more forgiving but not immune. A volume is a difference between two surfaces, so a constant vertical offset cancels out — as long as both surfaces carry the same kind of height. Mix one ellipsoidal surface with one orthometric surface and you introduce a systematic error across the whole site.
How the mistake usually happens
A drone survey is processed with GNSS control and comes out ellipsoidal. It is compared against an earlier survey that was levelled to benchmarks, so orthometric. Both are correctly georeferenced, both open without a warning, and the difference between them is contaminated by the geoid separation.
The tell-tale sign is a difference map that is uniformly biased — the whole site is "up" or "down" by a suspiciously round number, with the real changes sitting on top of that shift.
The habit that prevents it
Write down the vertical reference of every dataset when you receive it, in the file name or a project note. Elevation files rarely carry it in a way software can read, so the metadata has to be human.
Do any conversion once, at the source, with a geoid model appropriate for your region — then keep the whole project in that one reference.
STREAM computes elevations, slopes and volumes in whatever vertical reference your data already carries; it does not convert between ellipsoidal and orthometric heights. That makes the rule straightforward: bring in surfaces that already share one vertical reference, and every derived number will be consistent. STREAM will warn you when a layer's coordinate system does not match the project, but no software can detect a vertical reference that was never recorded.
Frequently asked questions
What is the difference between ellipsoidal height and orthometric height?
Ellipsoidal height is measured relative to a smooth mathematical ellipsoid model of the Earth, as used in satellite/GNSS positioning, while orthometric height is measured relative to the geoid, the gravity-defined surface roughly following mean sea level extended beneath the continents; the two can differ by tens of metres at the same location.
What is geoid separation?
Geoid separation (often written N) is the gap between ellipsoidal and orthometric height at a given location, related by orthometric height ≈ ellipsoidal height − N. It ranges from roughly −100 to +85 metres globally and varies across a country, so a single constant offset can't be applied except at small sites.
Why does mixing ellipsoidal and orthometric heights matter in terrain and GIS work?
Drainage, grading, flood levels and anything else driven by gravity must be computed on orthometric heights, or the design is simply wrong. Volumes are more forgiving since a constant offset cancels out between two surfaces of the same height type, but mixing one ellipsoidal surface with one orthometric surface introduces a systematic error across the whole site.
How does the ellipsoidal/orthometric height mix-up typically happen in practice?
A drone survey processed with GNSS control comes out ellipsoidal, while an earlier survey levelled to benchmarks is orthometric. Both are correctly georeferenced and open without any warning, so the geoid separation gets silently baked into the comparison, showing up as a uniformly biased difference map.
Sources
Keep one coordinate system per project in STREAM
Native Windows · fully offline · free beta