coordinate system - Difference between Geographic-3D and Geocentric CRS

I'll start with the OGP definitions:

• Geographic. A coordinate reference system based on a geodetic datum and using an ellipsoidal (including spherical) model of the Earth. This provides an accurate representation of the geometry of geographic features for a large portion of the Earth’s surface. Geographic coordinate reference systems can be two- or three-dimensional. A Geographic 2D CRS is used when positions of features are described on the surface of the ellipsoid through latitude and longitude coordinates; a Geographic 3D CRS is used when positions are described on, above or below the ellipsoid and includes height above the ellipsoid. [...]


• Geocentric. A coordinate reference system based on a geodetic datum that deals with the Earth’s curvature by taking the 3D spatial view, which obviates the need to model the curvature. The origin of a geocentric CRS is at the centre of mass of the Earth.

In practice, I would identify the difference between, say, a Geographic-3D and a Geocentric CRS in the fact that the latter has a central point in the 3D space from which angles and elevation are computed (no ellipsoid is needed, Earth is spherical?), whereas the former refers to the datum/ellipsoid, i.e. latitudes, longitudes and heights are computed with respect to the ellipsoid.

Am I saying something wrong or missing something? Thanks.

Answer

Geographic-3D relies on an ellipsoid (which can be a spheroid or a sphere), and locations are specified with latitude, longitude on the ellipsoid, and a distance above or below the surface of the ellipsoid.

Geocentric coordinates are 3D Cartesian coordinates, with the origin at the notional centre of the Earth (ellipsoid, geoid, or physical Earth), and points defined as X,Y,Z triplets. For most cartographic purposes, this is pretty useless because as the X and Y coordinates change, so your position above the Earth changes, as well as other fun non-spherical issues.

Think of them as the difference between 2D polar coordinates, and 2D Cartesian coordinates. They're both complete, unambiguous, and useful ways of expressing the location of something, depending on the problem you want to solve.