Jointly spatial-temporal representation learning for individual trajectories

Individual trajectories, containing substantial information on human-environment interactions across space and time, is a crucial input for geospatial foundation models (GeoFMs). However, existing attempts, leveraging trajectory data for various applications have overlooked the implicit spatial-temporal dependency within trajectories and failed to encode and represent it in a format friendly to deep learning, posing a challenge in obtaining general-purpose trajectory representations. Therefore, this paper proposes a spatial-temporal joint representation learning method (ST-GraphRL) to formalize learnable spatial-temporal dependencies into trajectory representations. The proposed ST-GraphRL consists of three compositions: (i) a weighted directed spatial-temporal graph to explicitly construct mobility interactions over both space and time dimensions; (ii) a two-stage jointly encoder (i.e., decoupling and fusion) to learn entangled spatial-temporal dependencies by independently decomposing and jointly aggregating space and time information; (iii) a decoder guides ST-GraphRL to learn explicit mobility regularities by simulating the spatial-temporal distributions of trajectories. Tested on three real-world human mobility datasets, the proposed ST-GraphRL outperformed all the baseline models in predicting movement spatial-temporal distributions and preserving trajectory similarity with high spatial-temporal correlations. We also explore how spatial-temporal features presented in latent space, validating that ST-GraphRL understands spatial-temporal patterns. This method is also transferable for general-purpose geospatial data representations for broad downstream tasks, as well advancing GeoFMs developing.