This point subclass is used for the storage of geodetic control points, where more information is available than simply x, y, z and t. The units for chi and eta are seconds of arc, and for n the units are metres. Consider a normal to the ellipsoid and a normal to the geoid (the plumb line) constructed such that they intersect. Project the geoid's normal onto a north-south plane containing the ellipsoid's normal.
chi is the angle between the projected geoid's normal and the ellipsoid's normal.
eta is the angle, as measured on a plane defined by projecting the geoid's normal onto the plane orthogonal to the first and also containing the ellipsoid's normal, between the second projection of the geoid's normal and the ellipsoid's normal.
n is the geoid undulation, the difference in metres between the geoid and the ellipsoid, measured along a normal to the ellipsoid.