With new technology, people can share information about everyday places they go; the resulting data helps others find and evaluate places. Recent applications like Dodgeball and Sharescape repurpose everyday place information: users create local place data for personal use, and the systems display it for public use. We explore both the opportunities --new local knowledge, and concerns --privacy risks, raised by this implicit information sharing. We conduct two empirical studies: subjects create place data when using PlaceMail, a location-based reminder system, and elect whether to share it on Sharescape, a community map-building system. We contribute by: (1) showing location-based reminders yield new local knowledge about a variety of places, (2) identifying heuristics people use when deciding what place-related information to share (and their prevalence), (3) detailing how these decision heuristics can inform local knowledge sharing system design, and (4) identifying new uses of shared place information, notably opportunistic errand planning.

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Although they have potential, to date location-based information systems have not radically improved the way we interact with our surroundings. To study related issues, we developed a location-based reminder system, PlaceMail, and demonstrate its utility in supporting everyday tasks through a month-long field study. We identify current tools and practices people use to manage distributed tasks and note problems with current methods, including the common "to-do list". Our field study shows that PlaceMail supports useful location-based reminders and functional place-based lists. The study also sheds rich and surprising light on a new issue: when and where to deliver location-based information. The traditional 'geofence' radius around a place proves insufficient. I nstead, effective delivery depends on people's movement patterns through an area and the geographic layout of the space. Our results both provide a comp elling demonstration of the utility of location-based information and raise significant new challenges for location-based information distribution.

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Many people use the Internet to search for geographically local information, with a growing number of websites dedicated to this task. However, it is not clear exactly how users integrate geographic search with content-based search, nor how to obtain reliable information about places in a geographic region. We created Sharescape, a map-based application in which information is contributed by community members. We conducted a user study to evaluate the utility of this means of obtaining information and to investigate how users integrate geographic and content-based search. Our results suggest that 1) maps create an implicit context in an interface that designers should honor, 2) communitymaintained information about local geography has important benefits over information mined from web sites, and 3) users often are not aware of the privacy implications of their actions, and therefore designers should incorporate special privacy safeguards.