CSM and triaxial

Within the current CSM v3.0.3, which added support for variable radii on an ellipse, we (Astro and the CSM WG) specifically did not include support for a triaxial body. The newly added ellipsoid code is available in Ellipsoid.c and Ellipsoid.h: https://github.com/sminster/csm/blob/master/Ellipsoid.h

Why no support for a triaxial body?

Paraphrased from Randy K.

At Astrogeology, We have never tried to base map projections on a triaxial ellipsoid solution. We only use the triaxial shape as a crude topographic model for rectifying images when a body is triaxial and no detailed topographic information is available. For such bodies, we use a spherical reference surface in map projections because it is usually closer to correct than a biaxial surface would be.

The other concern for using a triaxial body is that there is no one formula used to derive the radius of the reference sphere from the triaxial shape (it's not a simple average, usually it's an independent fit to the data). This can be seen in the IAU report (Archinal et al., 2009). Thus if you want to input the triaxial parameters and have the source code use the sphere, you really ought to enter 4 numbers (the original triaxial values and the best-fit sphere as generally defined by the IAU report). Currently we believe it is better to just to set up spherical references for these triaxial bodies. And if needed, we can separately create a "global topographic" model that reflects a triaxial shape. This is how very irregular bodies can also be handled.

REF: Archinal, B. A., M. F. A'Hearn, E. Bowell, A. Conrad, G. J. Consolmagno, R. Courtin, T. Fukushima, D. Hestroffer, J. L. Hilton, G. A. Krasinsky, G. Neumann, J. Oberst, P. K. Seidelmann, P. Stooke, D. J. Tholen, P. C. Thomas, I. P. Williams (2011), "Report of the IAU/IAG Working Group on Cartographic Coordinates and Rotational Elements of the Planets and Satellites: 2009," Celestial Mechanics and Dynamical Astronomy, v.109, Issue 2, pp. 101-135. URL: https://astrogeology.usgs.gov/groups/iau-wgccre