Augmented reality has been shown to be useful in many application areas such as maintenance skills, urban planning, interior design and entertainment, etc. The development of the mobile augmented reality in these last years is due to the evolution of the technology at various levels: from sensors (GPS, inertial sensor, etc.) to mobile devices (Tablet-PC, PDA, etc.). In this paper, we present a mobile augmented reality system dedicated to outdoor applications. It encompasses a localization process based on an assistance scheme and combining several data acquired from a hybrid sensor (camera, GPS and inertial sensor). The visualization and interaction are performed using a tablet-PC. This system has been tested in an application intended to geologists in order to monitor and supervise the restoration of a castle on a long term. This paper details various components of the system, some results that were obtained and how the whole application was evaluated through end-user.