Multi Activity Recognition Based on Bodymodel-Derived Primitives -- Activity Inference through Sequence Alignment -- Improving Location Fingerprinting through Motion Detection and Asynchronous Interval Labeling -- Using Context Annotated Mobility Profiles to Recruit Data Collectors in Participatory Sensing -- Location Diversity: Enhanced Privacy Protection in Location Based Services -- Nonvisual, Distal Tracking of Mobile Remote Agents in Geosocial Interaction -- Directional Beaconing: A Robust WiFi Positioning Method Using Angle-of-Emission Information -- Bluetooth Tracking without Discoverability -- Error Estimation for Indoor 802.11 Location Fingerprinting -- Activity Recognition from Sparsely Labeled Data Using Multi-Instance Learning -- A Framework for Creating and Using Maps of Privately Owned Spaces -- Daily Routine Recognition through Activity Spotting -- SofTOA: Software Ranging for TOA-Based Positioning of WLAN Terminals -- Position Estimation from UWB Pseudorange and Angle-of-Arrival: A Comparison of Non-linear Regression and Kalman Filtering -- An Ultrasonic 3D Positioning System Using a Single Compact Receiver Unit -- LOCK: A Highly Accurate, Easy-to-Use Location-Based Access Control System -- Characterizing the Space by Presenting Non-visual Information.

This book constitutes the refereed proceedings of the 4th International Symposium on Location and Context Awareness, LoCA 2009, held in Tokyo, Japan, in May 2009. The 18 revised full papers presented were carefully selected from 77 submissions. The papers describe original, previously unpublished research results, including new hardware platforms for sensing location and context, machine learning techniques for inferring user location and context from low-level sensor data, location and context representation, management, and distribution, and privacy policies and communication protocols for location and context information.