In landscape ecology spatial descriptors (or indices) can be used to characterize habitats. Some of these descriptors can be used for habitat's ranking; this ranking is very important for conservation purposes. We would like to show that two traditional descriptors, namely the compactness and interior-to-edge ratio can give contradictory results. Being the second one is a more relevant descriptor, we would like to propose to avoid the further use the compactness in habitat's ranking.