This paper presents a mobile Augmented Reality (AR) system called the SitePack supporting architects in visualizing 3D models in real-time on site. We describe how vision based feature tracking techniques can help architects making decisions on site concerning visual impact assessment. The AR system consists only of a tablet PC, a web cam and a custom software component. No preparation of the site is required and accurate initialization, which has not been addressed by previous papers on real-time feature tracking, is achieved by a combination of automatic and manual techniques.