What are the orientations of the XYZ axis in the Gravity component of GeographicLib?

Question

GeographicLib's Gravity utility computes the vector components gx, gy, and gz at a place, given its geodetic latitude, longitude, and altitude relative to the reference ellipsoid. I want to make sure I see this the way the author intended it, because to me this is not clearly described in the documentation. I asked the author but sofar got no reply.

1. The X component of the gravity vector points eastwards (positive direction) and is tangential to the parallel through the place.

2. The Y component of the gravity vector points northwards (positive direction) and is tangential to the meridian through the place.

3. The Z component of the gravity vector points upwards (positive direction) and passes through the place along the normal to the ellipsoid? This one in particular is not clearly stated in the docs.

Am I right?

Answer 1

The components of the gravity vector are expressed in an ENU (east, north, up) coordinate system. The up vector is normal to the ellipsoid (drop a perpendicular from the test point onto the ellipsoid).