Unless otherwise stated, the spacecraft altitude above Bennu is calculated as the distance between the s/c and the sub-spacecraft point.
The sub-spacecraft point is defined as the point where a straight line drawn from the s/c to the center of Bennu intersects the surface of Bennu.
The surface of Bennu is represented by topographic data contained in the "bennu_g_00880mm_alt_obj_0000n00000_v020.bds" DSK file created by the OSIRIS-REx Altimetry Working Group on 2020-04-14 with global coverage at 0.88 meter resolution.
The rotational and ellipsoidal shape constants for asteroid Bennu are obtained from bennu_v17.tpc NAIF's data file.
This model is currently used only for the calculations of the Post-TAG observation phase.
Here's an image of the Bennu shape model used to calculate the s/c altitude.
The heatmap shows the distance between a point on the surface of Bennu and the center of Bennu.
The popup label shows the longitude ("x"), the latitude ("y") and the distance ("z") in meters.
Here's the elevation of the Bennu surface above the reference triaxial ellipsoid (see next heatmap).
Here's the reference triaxial ellipsoid with radii: 283.065, 271.215, 249.720 m.
Here's an image of the Bennu shape model used to calculate the s/c altitude.