This communication focuses on the characterisation of a similarity measure between parts of 2D point clouds. This measure is defined thanks to the use of a general knowledge about real point clouds: they share a large amount of one-dimensional structures. These structures can be represented into a unified manner with a new type of primitives; then, we set the link between the existence of common information between parts of point clouds and the geometric relations of their primitives. Thus, we define a similarity measure that is rotationally invariant, and an algorithm to compute it.