A privacy-preserving semantic annotation framework using online social media

Semantic annotation framework that allows enriching locations or trajectories with semantic abstractions of the raw spatiotemporal data benefits understanding the semantic behavior of moving objects. Existing semantic annotation approaches mainly analyze specific parts of a trajectory, e.g. stops, in association with data from 3rd party geographic sources, e.g. (POI) points-of-interest, road networks. However, these semantic resources are static thus miss important dynamic event information. Recent location-based social networking provides a new dynamic and prevalent source of human activity data that can be a potential semantic resource for annotation. However, using the large-scale spatiotemporal data from online social media gives rise to privacy concerns. This paper thus presents a privacy-preserving semantic annotation framework P-SAFE that (i) identifies dynamic region of interest (DRI) from large-scale data provided by location based social networks whilst labelling of DRI into appropriate categories derived from spatial and temporal features of geotags, (ii) aligns trajectories to a set of DRI and enriches trajectories with semantics annotation derived from aligned DRI via THMM model, and (iii) embeds robust privacy-preserving mechanisms under differential privacy in each stage that accesses to raw data. P-SAFE approach tackles the privacy and utility trade-offs for meaningful geographic regions identification and labeling as well as trajectory semantic annotation under differential privacy whilst combining them into a single task. We demonstrate the effectiveness of P-SAFE approach on a dataset of large-scale geotagged tweets and a benchmark trajectory dataset for DRI construction and trajectory semantic annotation evaluation. The experimental results illustrate that P-SAFE not only provides robust privacy guarantees but remains approximate 45–56% accuracy for meaningful geographic regions labelling and 62–76% accuracy for trajectory semantic annotation.