Why vertical datums change altitude numbers

A defined reference

A height number is a distance from a realised surface, not an absolute label.

A vertical datum supplies the reference from which elevations or depths are stated. It lets measurements made at different places and times belong to one height system. Without that reference, 450 metres is incomplete whenever another dataset uses a different zero.

The datum is not merely a note added after measurement. It belongs to the coordinate. Changing the datum can change the numerical height assigned to the same physical point. The ground has not moved; the reference has.

Some datums are geometric and use a reference ellipsoid. Others are gravity-based and support orthometric height. Tidal datums serve water-level and coastal purposes. A conversion must match the type, region and realisation involved. There is no single worldwide correction that turns every device height into every mapped elevation.

Two reference surfaces

The ellipsoid is mathematical. The geoid follows Earth's gravity field.

A reference ellipsoid is a smooth mathematical model used to express latitude, longitude and ellipsoidal height. GNSS positioning naturally works in a three-dimensional reference frame associated with an ellipsoid. The geoid is an equipotential surface of Earth's gravity field and is irregular relative to that ellipsoid.

Orthometric height is measured from a gravity-based reference surface towards the point, broadly matching the practical idea of elevation above sea level. Ellipsoid height and orthometric height are related through geoid height, often written as h = H + N. The signs and compatible models matter. The equation does not authorise mixing an arbitrary WGS 84 value with a local geoid correction.

The geoid is often described by imagining calm oceans extended beneath the continents, but it is not today's shoreline or an average of the nearest tide. It is a model of gravitational potential.

Great Britain

GNSS and familiar OS mapping use connected but different systems.

Modern precise GNSS positioning in Great Britain is associated with ETRS89. Familiar Ordnance Survey mapping uses OSGB36 for horizontal coordinates, while mainland heights are related to Ordnance Datum Newlyn. These are not alternative names for one three-dimensional coordinate system.

Ordnance Survey provides OSTN15 and OSGM15 transformation models to connect GNSS positions with the national grid and mean-sea-level-based height system. OSGM15 models the separation needed for the vertical component. This is a spatially varying relationship, not a fixed number that should be typed into every route.

Other territories and islands can use different height datums. A route that crosses datasets should retain the exact coordinate reference information supplied by each source rather than assuming that a British label covers every area.

Diagnose the disagreement

Preserve the original value and trace every conversion.

1. Name

   Identify the source datum and height type.

   Look for ellipsoidal, orthometric, tidal or local references in metadata and documentation.

2. Match

   Choose a transformation valid for the place and systems.

   A national geoid or grid model has a defined extent and compatible inputs.

3. Convert

   Record the model and version used.

   This keeps the converted copy reproducible instead of silently overwriting its provenance.

4. Check

   Compare a trusted control or mapped point where appropriate.

   A remaining local mismatch may come from the source measurement or terrain model rather than the datum.

Use beacen Support when the problem concerns a product-specific display.

Core GPX elevation values do not name a vertical datum, so an exporter without supporting documentation may leave the exact conversion unknowable. Keep that uncertainty attached to every derived value.