Poster advertisement has a long tradition and is transforming rapidly into digital media. In this paper we provide an overview of how sensing can be used to create online and up to date information about potential viewers. We assess what application domains can benefit from continuous monitoring of visitors. As measuring with simple sensors is inherently error prone we suggest the notion of comparative advertising power which compares the number of potential viewers in different locations. We address user acceptance and privacy concerns and show technical mechanism to increase privacy.