We provide details about the procedure employed for the three-dimensional geological modelling of the lobes of comet Churyumov-Gerasimenko (67P). The two lobes of 67P are characterized by well visible terraces and elongated cliffs that revealed a pervasive layering arranged in an onion-like fashion. None of the layers can be traced for a continuity large enough to provide a means of stratigraphic correlation. Therefore, an explicit modelling approach of the layered structure is not easily applicable. We show how a very simple modelling strategy based on implicitly-defined analytical surfaces (spheres or ellipsoids), and requiring very limited operator decision-making, can be successfully applied to produce a geological model that easily fits the available scattered attitude observations. Our formulation has the advantage of providing a small set of parameters with a precise geometrical meaning that can be compared with other parameters of the lobes (i.e. center of ellipsoids and center of mass). The presented method originates outside commercial geological modelling software packages and required devising some ad-hoc solutions for the visualization of the resulting models in comparison with observations made on OSIRIS images. Some extracts of the visualization code, helpful for applying this procedure in similar operational contexts, are also presented.
