A method of calculating and encoding a score of semantic and spatial similarity between concepts of an ontology stored in hierarchically numbered trellis form, in which the score (NSS.sub.ij) is calculated and encoded (1) for each concept (C.sub.i) with respect to a central concept (C.sub.j), taken two-by-two, by convergence relative to their common semantic characteristics respectively by separation according to their distance. The method is applicable to the consultation of ontologies representative of knowledge in the technical or non-technical domain.