GPS trajectories are the essential foundations for many trajectory-based applications. Most applications require a large number of high sample rate trajectories to achieve a good performance. However, many real-life trajectories are collected with low sample rate due to energy concern or other constraints. We study the task of trajectory recovery in this paper as a means to increase the sample rate of low sample trajectories. Most existing works on trajectory recovery follow a sequence-to-sequence diagram, with an encoder to encode a trajectory and a decoder to recover real GPS points in the trajectory. However, these works ignore the topology of road network and only use grid information or raw GPS points as input. Therefore, the encoder model is not able to capture rich spatial information of the GPS points along the trajectory, making the prediction less accurate and less spatial consistent. In this paper, we propose a road network enhanced transformer-based framework, namely RNTrajRec, for trajectory recovery. RNTrajRec first uses a graph model, namely GridGNN, to learn the embedding features of each road segment. It next develops a spatial-temporal transformer model, namely GPSFormer, to learn rich spatial and temporal features along with a Sub-Graph Generation module to capture the spatial features for each GPS point in the trajectory. It finally forwards the outputs of encoder model to a multi-task decoder model to recover the missing GPS points. Extensive experiments based on three large-scale real-life trajectory datasets confirm the effectiveness of our approach.

With widespread deployment of GPS devices, massive spatiotemporal trajectories became more accessible. This booming trend paved the solid data ground for researchers to discover the regularities or patterns of human mobility. However, there are still three challenges in semantic pattern extraction including semantic absence, semantic bias and semantic complexity. We invent and apply a novel data structure namely City Semantic Diagram to overcome above three challenges. First, our approach resolves semantic absence by exactly identifying semantic behaviours from raw trajectories. Second, the design of semantic purification helps us to detect semantic complexity from human mobility. Third, we avoid semantic bias using objective data source such as ubiquitous GPS trajectories.

Hamming space retrieval, allowing for retrieval within a fixed Hamming radius rather than scanning all instances linearly, has gained widespread attention for retrieving nearest neighbors at a fixed computational cost. Current models have focused only on shared-label correlations as the label-wise semantics while ignoring the potential multi-hop constraints, which are the unique constraints in Hamming space retrieval under multi-label conditions. Instances with multi-dimensional labels form a multi-hop correlation graph rather than several clusters in a single-label scenario. So there are potential distance constraints between dissimilar instances connected through multi-hop correlations. Existing models blindly expanding the dissimilar instances will break the multi-hop correlations and finally disrupt the hashing consistency of the shared-label instances. This paper first defines the multi-hop correlations to address these challenges with the multi-hop preserving strategy for dissimilar instances to help the Hamming distance converge to the expected range. Meanwhile, we introduce a Wasserstein-1-distance-based loss to reduce information loss while encoding compactly. We evaluate our proposed model on three commonly used datasets with extensive experiments to demonstrate that our model achieves significant improvements over existing methods.

Hashing plays an important role in the content-based retrieval of multimedia data. Existing methods focus on designing various joint-optimization strategies to preserve the similarity relationships between different modalities and improve the performance of cross-modal retrieval. However, when the intrinsic information of any modality is significantly insufficient compared with others, the final hash space will collapse to the trivial space due to the joint training. Compared with inter-modal semantic alignment, directly aligning the semantics of each modality with the label-wise semantics can obtain higher-quality common semantic spaces, and only a pair-wise alignment between each space can obtain a unified representation. Following this idea, we design a general hash generation framework for uni-modal embedding and directly align the hashing code from different modalities using a pair-wise loss, further improving the retrieval performance within the less-semantic modality. Within this framework, we introduce two optimizations to penalize dissimilar items. First, with a Gaussian distribution to describe the hashing semantic distribution, JS-divergence is introduced to keep the consistency of label-wise semantics and hashing similarity. Then the attention mechanism is used for hard-sample re-weighting to learn fine-grained distribution alignment further. We conduct extensive experiments on three public datasets to validate the enhancements of our work.

Searching spatial objects is a fundamental task in spatial services such as online maps. Traditional search methods are based on filtering conditions, burdening users to specify their requirements. This paper focuses on spatial search via examples. Particularly, the user can specify an example, which is a set of objects of interest, and the purpose is to find a list of results, each containing a set of objects with similar properties to the given example. We conducted a user study, showing that a search interface based on examples can effectively complement existing approaches. However, the existing example-based search is not scalable, hindering its applications to larger datasets. To address this challenge, we propose two new algorithms, namely HSP and LORA, to efficiently answer example-based spatial queries. HSP is an algorithm based on a hierarchical partitioning of the search space, and it achieves up to 20 times faster than the state-of-the-art algorithm. LORA further improves the efficiency, running up to 5000 times faster than the state-of-the-art algorithm. We present a systematic evaluation to demonstrate the efficacy of our algorithms.

With widespread deployment of GPS devices, massive spatiotemporal trajectories became more accessible. This booming trend paved the solid data ground for researchers to discover the regularities or patterns of human mobility. However, there are still three challenges in semantic pattern extraction including semantic absence, semantic bias and semantic complexity. In this paper, we invent and apply a novel data structure namelyCity Semantic Diagramto overcome above three challenges. First, our approach resolves semantic absence by exactly identifying semantic behaviours from raw trajectories. Second, the design of semantic purification helps us to detect semantic complexity from human mobility. Third, we avoid semantic bias using objective data source such as ubiquitous GPS trajectories. Comprehensive and massive experiments have been conducted based on real taxi trajectories and points of interest in Shanghai. Compared with existing approaches,City Semantic Diagramis able to discover fine-grained semantic patterns effectively and accurately.

Spatio-temporal trajectory classification is a fundamental problem for location-based services with many real-world applications such as travel mode classification, animal mobility detection, and location recommendation. In the literature, many approaches have been proposed to solve this classification task including deep learning models like LSTM recently for sequence classification. However, these approaches fail to consider both spatial and temporal interval information simultaneously, but share some common drawbacks: omitting either the spatial information or the temporal interval information out. Some models like Time-LSTM, have been proposed to handle the temporal interval information for spatio-temporal trajectories, but they do not take into account the spatial information. Note that, considering both spatial and temporal interval information is crucial for spatio-temporal data mining in order not to miss any spatio-temporal pattern. In this study, we propose a trajectory classifier called Spatio-Temporal GRU to better model the spatio-temporal correlations and irregular temporal intervals prevalently present in spatio-temporal trajectories. We introduce a novel segmented convolutional weight mechanism to capture short-term local spatial correlations in trajectories and propose an additional temporal gate to control the information flow related to the temporal interval information. Performance evaluation demonstrates that our proposed model outperforms popular deep learning approaches for the travel model classification problem.

Trajectory outlier detection is a fundamental building block for many location-based service (LBS) applications, with a large application base. We dedicate this paper on detecting the outliers from vehicle trajectories efficiently and effectively. In addition, we want our solution to be able to issue an alarm early when an outlier trajectory is only partially observed (i.e., the trajectory has not yet reached the destination). Most existing works study the problem on general Euclidean trajectories and require accesses to the historical trajectory database or computations on the distance metric that are very expensive. Furthermore, few of existing works consider some specific characteristics of vehicles trajectories (e.g., their movements are constrained by the underlying road networks), and majority of them require the input of complete trajectories. Motivated by this, we propose a vehicle outlier detection approach namely DB-TOD which is based on probabilistic model via modeling the driving behavior/preferences from the set of historical trajectories. We design outlier detection algorithms on both complete trajectory and partial one. Our probabilistic model-based approach makes detecting trajectory outlier extremely efficient while preserving the effectiveness, contributed by the relatively accurate model on driving behavior. We conduct comprehensive experiments using real datasets and the results justify both effectiveness and efficiency of our approach.

More and more advanced technologies have become available to collect and integrate an unprecedented amount of data from multiple sources, including GPS trajectories about the traces of moving objects. Given the fact that GPS trajectories are vast in size while the information carried by the trajectories could be redundant, we focus on trajectory compression in this article. As a systematic solution, we propose a comprehensive framework, namely, COMPRESS ( Comprehensive Paralleled Road-Network-Based Trajectory Compression ), to compress GPS trajectory data in an urban road network. In the preprocessing step, COMPRESS decomposes trajectories into spatial paths and temporal sequences, with a thorough justification for trajectory decomposition. In the compression step, COMPRESS performs spatial compression on spatial paths, and temporal compression on temporal sequences in parallel. It introduces two alternative algorithms with different strengths for lossless spatial compression and designs lossy but error-bounded algorithms for temporal compression. It also presents query processing algorithms to support error-bounded location-based queries on compressed trajectories without full decompression. All algorithms under COMPRESS are efficient and have the time complexity of O (| T |), where | T | is the size of the input trajectory T . We have also conducted a comprehensive experimental study to demonstrate the effectiveness of COMPRESS, whose compression ratio is significantly better than related approaches.

Vehicle trajectories are one of the most important data in location-based services. The quality of trajectories directly affects the services. However, in the real applications, trajectory data are not always sampled densely. In this paper, we study the problem of recovering the entire route between two distant consecutive locations in a trajectory. Most existing works solve the problem without using those informative historical data or solve it in an empirical way. We claim that a data-driven and probabilistic approach is actually more suitable as long as data sparsity can be well handled. We propose a novel route recovery system in a fully probabilistic way which incorporates both temporal and spatial dynamics and addresses all the data sparsity problem introduced by the probabilistic method. It outperforms the existing works with a high accuracy (over 80%) and shows a strong robustness even when the length of routes to be recovered is very long (about 30 road segments) or the data is very sparse.

Map data are widely used in mobile services, but most maps might not be complete. Updating the map automatically is an important problem because road networks are frequently changed with the development of the city. This paper studies the problem of recovering missing road segments via GPS trajectories, especially low sampled data. Our approach takes the GPS noise into consideration and proposes an effective self-adaptive algorithm. Besides, we propose theoretical models behind all the important parameters to enable self-adaptive parameter setting. To the best of our knowledge, this is the first work that addresses the parameter setting issue successfully to make sure our approach is free of parameter-tuning. In addition, we also propose a quantitative evaluation method for map updating problem. The result shows our algorithm has a much better performance than the existing approaches.

Spatial queries such as range query and kNN query in road networks have received a growing number of attention in real life. Considering the large population of the users and the high overhead of network distance computation, it is extremely important to guarantee the efficiency and scalability of query processing. Motivated by the scalable and secure properties of wireless broadcast model, this paper presents an air index called Network Partition Index (NPI) to support efficient spatial query processing in road networks via wireless broadcast. The main idea is to partition the road network into a number of regions and then build the index to carry some pre-computation information of each region. We also propose multiple client-side algorithms to facilitate the processing of different spatial queries such as kNN query, range query and CNN query. A comprehensive experimental study has been conducted to demonstrate the efficiency of our scheme.

Location data becomes more and more important. In this paper, we focus on the trajectory data, and propose a new framework, namely PRESS ( Paralleled Road-Network-Based Trajectory Compression ), to effectively compress trajectory data under road network constraints. Different from existing work, PRESS proposes a novel representation for trajectories to separate the spatial representation of a trajectory from the temporal representation, and proposes a Hybrid Spatial Compression (HSC) algorithm and error Bounded Temporal Compression (BTC) algorithm to compress the spatial and temporal information of trajectories respectively. PRESS also supports common spatial-temporal queries without fully decompressing the data. Through an extensive experimental study on real trajectory dataset, PRESS significantly outperforms existing approaches in terms of saving storage cost of trajectory data with bounded errors.

Aggregate nearest neighbor query, which returns a common interesting point that minimizes the aggregate distance for a given query point set, is one of the most important operations in spatial databases and their application domains. This paper addresses the problem of finding the aggregate nearest neighbor for a merged set that consists of the given query point set and multiple points needed to be selected from a candidate set, which we name as merged aggregate nearest neighbor(MANN) query. This paper proposes an effective algorithm to process MANN query in road networks based on our pruning strategies. Extensive experiments are conducted to examine the behaviors of the solutions and the overall experiments show that our strategies to minimize the response time are effective and achieve several orders of magnitude speedup compared with the baseline methods.

The ACM SIGSPATIAL Cup 2013 is about geo-fencing, a problem of determining whether sampled location points fall inside or within a certain distance from virtual perimeters for real-world geographic areas. This paper describes one of the winning submissions of the competition. The approach applies two novel techniques, namely Trajectory Partitioning (TP) and Boundary Simplification (BS). TP partitions the movement trajectory of an object into segments and processes the points within the same segment simultaneously. BS simplifies the polygons thus accelerates the geo-fencing process.

Due to the proliferation of Location-Based Service and popularity of online geo-tagged web pages, spatial keyword search has attracted significant attention from both academic and industrial communities. In this paper, we study the problem of finding the nearest aggregate point from multiple query points travelling through a set objects described by a given set of keywords, as well as the optimal routes from the query points to the aggregate point. This problem is defined as the Aggregate Keyword Routing (AKR) Query. We devise an exact algorithm to AKR query based on ellipse pruning. Next we propose an efficient approximate algorithm for AKR: Center Based Assignment (CBA). The performance of the proposed algorithms are evaluated with real data, the results demonstrate the efficiency and the effectiveness.

The ACM SIGSPATIAL Cup 2012 is about map matching, a problem of correctly matching a sequence of GPS sampling points to the roads on a digital map. This paper describes one of the winning submissions of the competition. The approach applies multi-threading technology to map matching in order to reduce running time and we propose an improvement to the Hidden Markov Model (HMM) map matching algorithm.

Wireless broadcast provides a scalable and secure spatial data dissemination approach for geographical applications in wireless mobile environments. Among various location-based services, the shortest path query on road networks is one of the most popular and essential services in our daily life. In this paper, we propose an energy-efficient scheme for on air shortest path query processing on road networks, which leverages an elaborate air index called BagIndex based upon the novel Hilbert-based heuristic tree decomposition for the road networks. Experimental results show that the proposed approach incurs less energy consumption on both communication and computation than the previous schemes.

Aggregate nearest neighbor (ANN) query returns a common interesting data object that minimizes an aggregate distance for multiple query points. In this paper, we propose a novel approach to efficiently process ANN queries in road networks. This approach includes two processes: initializing process and pruning process, which are both based on computing kNN of query points in network Voronoi diagram. Experimental results show that our approach outperforms existing approaches on both response time and page accesses.

Wireless sensor networks have emerged recently as an effective way of gathering useful information from areas of interest. Prolonging the network lifetime has become the primary concern in data gathering due to the limited battery power of sensors. An underlying assumption of most existing work is that all the sensors are working simultaneously during the whole process of data gathering. In this paper, we introduce a new definition of the network lifetime in view of the coverage condition, and then investigate the coverage-based maximum lifetime data gathering problem theoretically. Two critical issues are how to selectively organize part of sensors to work and how to execute routing in a single working set. Considering both of these two problems together, we propose the algorithm CODAG. CODAG employs the tree structured routing scheme and enables the routing effects of two different sets theoretically comparable. Based on the maximal increased weight in the routing tree, it divides all the sensors into disjoint cover sets without redundant ones, which in turn leads to the better routing effect. Experimental results show that our algorithm has the good performance for prolonging the lifetime of the whole sensor network.