An underactuated nonholonomic Dubins-vehicle-like robot with a lower-limited turning radius travels with a constant speed in a plane, which hosts unknown complex objects. The robot has to approach and then circumnavigate all objects, with maintaining a given distance to the currently nearest of them. So the ideal targeted path is the equidistant curve of the entire set of objects. The focus is on the case where this curve cannot be perfectly traced due to excessive contortions and singularities. So the objective shapes into that of automatically finding, approaching and repeatedly tracing an approximation of the equidistant curve that is the best among those trackable by the robot. The paper presents some geometric facts that are in demand in research on reactive tight circumnavigation of group objects in the delineated situation.