Use of pattern matching algorithm for location detection in a wireless LAN is well known in literature. It allows to achieve good performances in terms of localisation errors, but it may require a very long and time-consuming start-up phase, during which a set of measurements of the radio signal strength values received in different positions inside the area involved in the localisation, must be taken. The complexity of this procedure increases with the size of the area involved in the localisation; this clearly happens for large outdoor areas, where a huge set of measurements must be taken during the start-up phase. The paper aims to present an approach for localisation in outdoor areas covered by a wireless network; this approach is still based on the use of pattern matching algorithm, but use of a signal propagation model during the start-up phase has been introduced in order to reduce its complexity. Performances of the proposal have been evaluated for outdoor localisation and the main results achieved will be shown in the paper.