Heterogeneous Social Linked Data Integration and Sharing for Public Transportation

Solid (social linked data) technology has made signiﬁcant progress in social web applications developed, such as Facebook, Twitter, and Wikipedia. Solid is based on semantic web and RDF (Resource Description Framework) technologies. Solid platforms can provide decentralized authentication, data management, and developer support in the form of libraries and web applications. However, thus far, little research has been conducted on understanding the problems involved in sharing public transportation data through Solid technology. It is challenging to provide personalized and adaptable public transportation services for citizens because the public transportation data originate from diﬀerent devices and are heterogeneous in nature. A novel approach is proposed in this study, in order to provide personalized sharing of public transportation data between diﬀerent users through integrating and sharing these heterogeneous data. This approach not only integrates diverse data types into a uniform data type using the semantic web but also stores these data in a personal online data store and retrieves data through SPARQL on the Solid platform; these data are visualized on the web pages using Google Maps. To the best of our knowledge, we are the ﬁrst to apply Solid in public transportation. Furthermore, we conduct performance tests of the new C2RMF (CSV to RDF Mapping File) algorithm and functional and non-functional tests to demonstrate the stability and eﬀectiveness of the approach. Our results indicate the feasibility of the proposed approach in facilitating public transportation data integration and sharing through Solid and semantic web technologies.


Introduction
Data sharing for public transportation could address the complex city challenges and implement the efficient and effective management of city spaces, and it could be an effective way to reduce pollution, carbon emissions, global climate change, and even manage health risks in smart cities [1]. Data sharing can involve data such as bus routes, live bus arrival times, train routes, car parking availabilities, and live traffic routes. In addition, a range of data acquisition methods exists. Furthermore, there exist different system sources and data storage formats.
is makes the data composition unstructured, which causes heterogeneity in the data composition. Furthermore, heterogeneous data make it difficult to share public transportation data [2][3][4][5][6].
To address these challenges, many studies have been made over the past years. Some researchers tried to integrate heterogeneous data by Web API [7,8]. ey built a virtual integration way in which a unified query interface is provided to a large number of heterogeneous data sources, but this method is limited to popular programming languages and cannot get over the challenges of evolving API that need to modify the existing client codebase [9].
To realize the unified management of heterogeneous data in data applications, one method is to use ETL tools to process heterogeneous data in a unified form. However, this method is costly, and changes in data and analysis requirements will cause the original ETL process to fail [10].
Other studies have shown that the semantic web is "a common framework that allows data to be shared and reused across application, enterprise, and community boundaries [11]." Semantic web technology [12] promotes data integration from multiple heterogeneous data sources, enables the development of information-filtering systems, and supports knowledge discovery tasks. It helps to integrate diverse data and share these data through the semantic web. However, most of the content on the World Wide Web has not yet been marked up to comply with the semantic web specification. erefore, how to automatically add tags that conform to the semantic web specification to the existing web content is one of the difficult problems facing the practical application of the semantic web.
To provide personalized sharing of public transportation data between different users through integrating and sharing these heterogeneous data, a new approach is proposed in this study.
e proposed approach mainly has three key elements: a data processing method, a CSV to RDF (Resource Description Framework) Mapping File algorithm, and a system framework. Firstly, in order to integrate heterogeneous data, a data processing method is proposed. is data processing method includes the process of collecting, organizing, and normalizing data. Data processing is the basic element of data and data system management and involves the management process of the full life cycle of data. Based on the above point, the main focus is on unstructured data integration. C2RMF (CSV to RDF Mapping File) algorithm is designed, which mainly converts CSV files into RDF files and converts them into a unified data format for subsequent processing. ird, to share these integrated heterogeneous data, a system framework is proposed.
is framework mainly performs unified data storage, query, and visualization of the unified transformed files. e main contributions of this study are summarized as follows: (i) e proposed data processing method involves collecting, integrating, generating, and sharing the heterogeneous public transportation data. (ii) Based on the proposed method, the newly developed C2RMF algorithm for integrating heterogeneous data is proposed. (iii) A novel system framework for integrating and sharing these heterogeneous public transportation data is proposed. e remainder of this paper is organized as follows. Section 2 describes related work. Section 3 describes the proposed methodology in detail. Section 4 describes the system framework. Section 5 describes the system implementation process. e next section describes the experimental environment, data, process and results, and analysis. e conclusions and recommendations are given in the final section [13].

RDFS and OWL.
Integration and sharing of heterogeneous public transportation data is an important problem in recent years. Previous research and related technologies had solved users' needs to a certain extent, but there are still some problems [7][8][9][10][11][12]. Recently, RDF (Resource Description Framework) has been widely used to expose, share, and connect pieces of data on the web. RDF is a W3C recommendation graph database model. RDF provides a model of nodes and relationships and is a more general model for the World Wide Web. RDF includes a range of statements, with each statement describing a resource. Every statement includes three parts: a subject, a predicate, and an object. e subject represents the resource; the predicate mainly represents a relationship between the subject and object and the property of that resource; the object represents the value of that property.
Although RDF provides the ability to create a graph of data, it is the RDF schema (RDFS) that supplies the building blocks for more complex vocabularies by enabling the definition of data types and structures within an RDF graph [14]. RDFS provides "all that is needed for interoperability of the vast amount of data on the web [15]." e RDFS can define a set of word sets that can be clearly described based on RDF resources and can be used to describe the subclass or superclass relation, subattribute or superattribute, domain and range of attributes, and instance constraints of the class. Another RDF data model named OWL (Web Ontology Language), which is a W3C recommendation, is a family of knowledge representation languages for authoring ontologies. It can describe more complex logical relationships between concepts. OWL extends RDFS and provides a more descriptive schema layer that can be used where the basic definitions afforded by RDFS are not expressive enough [16].
With the help of the data interconnection network constructed by RDFS and Web Ontology Language (OWL), they are combined with different heterogeneous datasets published on the web. However, they often lack a unified data operation mode for information storage and retrieval of unstructured data.

Social Linked Data.
Solid is also proposed to provide important capabilities for web-based data-sharing systems for public transportation as Solid is based on semantic web and RDF technologies, and it provides a unified data operation mode. In Solid, every user can store their dataset in an online storage space called a personal online data store (pod). Pods can be deployed on personal servers or on public servers by other pod providers. Application data in Solid are stored in documents that are identified by a Uniform Resource Identifier (URI) [17].
Pod refers to a personal online data store that is used to store user data on a Solid platform. It is possible for a user to have more than one pod [18]. A user can select diverse pod providers because Solid applications can operate with any pod server without the limitation of service provider and location. Diverse pod providers can provide diverse degrees of security, availability, and reliability. Different pods can access their data resources and deliver content to each other [17].
In summary, Solid is a web decentralization technology, and a Solid platform can provide a relatively effective and safe means of executing web applications. However, Solid technology still mainly focuses on social media networks. Few studies have considered using Solid in the transportation field. To fill this gap, this study applied Solid technology to public transportation data sharing based on the semantic web, and a new alternative approach was proposed to integrate and share these heterogeneous data.

Methodology
e methodology used in this study, presented in Figure 1, analyzed the system functional and non-functional requirements by analyzing the capability and usability of diverse public transportation apps. Next, the system design is given, and the system is implemented based on the system requirements and data processing. Finally, recommendations were devised through experiments and analysis of results.

Requirement Analysis Based on Comparison of Capability and Usability of Public Transportation Apps.
e system requirements were collected based on the capability of diverse public transportation apps compared with the usability of diverse public transportation apps. e public transportation apps in this study were categorized into four groups based on their capabilities and usability. e first group includes minicab apps, including Uber and Aqua cars. e second group includes train and coach apps including Trainline, Virgin Trains, and National Express and bus apps such as First Bus and UK Bus. e third group includes car parking apps such as JustPark and YourParkingSpace, and the last group includes Google and the system. e capabilities and usability of these apps are compared below (see Tables 1 and 2, respectively).
Defining attributes of usability, saving favorite routes, and recent searches and routes, as shown in Table 2, belong to an efficiency attribute because they are related to the accuracy and completeness with which users achieve goals. Sending the selected route to the phone, suggesting an appropriate route based on what the user needs, and sharing an appropriate route to car parking is part of a satisfaction attribute because they have a positive effect on the app usage. Help and FAQ functions make it easy to learn to use apps so that the user can rapidly start getting work done through the app, which is the learnability attribute. Social capabilities allow users to share experiences, including comments, pictures, and videos, and interact with other tourists. It is interesting for users to improve the use of a recommender during a visit to a particular place [19].

Functional and Non-Functional Requirements.
User requirements can be divided into two types: functional and non-functional requirements [20]. Functional requirements can be elicited directly from a user through software feature requests [21]. e non-functional requirement can be defined as part of the attributes of the system [20]. e functional requirements of the new system are listed in Table 3 (functional requirement table), and the non-functional requirements of the new system are listed in Table 4 (non-functional requirement table).

Proposed Data Processing
Method. e proposed data processing method has six key elements: data collected, data integrated and generated, data stored, data searched, and data shared. ese elements collect data from different data resources and transfer a uniform RDF model and then save the pod server on the Solid platform, and finally share these data through a web application (see Figure 2).
First, data are collected; the public transportation data are collected from tracking devices and sensors that are located in the entire city and installed on cameras and mobile phones [22]. ese public data are referred to as open data because they can be freely used and redistributed by someone either for or at marginal cost through existing web applications, for example, data.gov.uk [23]. During the data collection phase, diverse data sources will be collected through different data sources, such as the data of the Uber system [24], the data of the National Express system [25], and the data of car parking systems [26].
Second, we discuss data integrated and generated elements.
ere are diverse open data sources in public transportation, for example, National Express is XML; Uber and car parking datasets are structured data like CSV; and Trainline data are unstructured data, like TXT. erefore, it is necessary to consider heterogeneous datasets and translate them into a uniform data type. An appropriate semantic model can supply an interoperable representation of data [27]. erefore, semantic web technologies are suggested to address these requirements because they will supply the necessary capabilities for public transportation dataset unification of different open datasets. Each data source needed a diverse method to be extracted and changed into a uniform RDF data model because the RDF data model makes it easier to integrate system data than traditional data models, for example, relational data [28].
ird, data are stored; RDF is a graphics-based data model that can represent any data structure. RDFS can be defined within the RDF so that data structures and types can be adapted as the application demands. RDFS and OWL are programs that change diverse data types into a uniform RDF, adding a semantic mark-up that describes the meaning of each data item. Different data were unified in the RDF and stored in the pod server on the Solid platform, which is a new method because Solid technology was first used in the public transportation area. Solid technology allows users to have full control of their data, including access control and storage location.
Fourth is the data searched element: there are two possible methods for data searching on a Solid platform. One is the RESTful method in terms of the LDP [29]. Another is the SPARQL query method [18]. In a Solid platform, all pod servers must support the LDP, while some servers may optionally support SPARQL.
Finally, we have the data shared element: based on the above steps, users will have access to these data based on their different authority levels, such as access to the entire dataset or only part of the dataset and access to modifying data and comments after data are shared on the website. Figure 2 describes a complete process in which data is collected, integrated, stored, and shared. Each functional capability is defined as a separate function, permitting separate programming and evolution, as shown in Figure 2, which derives from architectures in Slogger: a profiling and analysis system based on semantic web technologies [30]. Figure 3 shows the system framework for implementing a web-based data-sharing system, including transformers, semantic web query algorithms, and visualization technology. is new proposed system framework for implementing public transportation data sharing is mainly based on Solid.

C2RMF (CSV to RDF Mapping File) Algorithm.
To change the CSV data of car parking into RDF for integrating heterogeneous data, the new C2RMF (CSV to RDF Mapping File) algorithm was developed. Converting CSV files to RDF files based on mapping files is currently the main way including the C2RMF algorithm. Some existing tools or ways are accomplished for CSV to RDF including our proposed algorithm. But these tools use different mapping techniques, and it is difficult to use and share these mapping engines. Furthermore, most of them lack the use of semantic web technology and W3C recommended standards [31].
C2RMF algorithm is fully in accordance with W3C recommendation [32], a Java-based method which converts CSV data into RDF. If no specific format is provided, the result is serialized as a TURTLE file by default. e algorithm can convert CSV files into RDF files and also can change some structured data of public transportation into RDF, adding a semantic mark-up that describes the meaning of each data item. e algorithm can convert each row of the input CSV data into a new instance of a uniform RDF class. Each value in the column of the source CSV is transformed into a new triple where each key represents a column position in the source, CSV stands for the subject, each property depending on the name of the column header stands for the predicate, and each value of the column stands for the object. e algorithm is also entirely customizable to meet specific user requirements in terms of the mapping file.
Define the RDF graph. Let M and N be a finite set of uniform resource identifiers and literals. A tuple (s, p, o) ∈M × M × (M∪N) can be called an RDF triple. Each RDF triple t � (s, p, o) indicates that resource s and resource o have a relationship p, where s, p, and o represent a subject, a predicate, and an object, respectively, and thus a finite set of triples is called an RDF graph. C2RMF algorithm is shown in Table 5. Figure 4 shows the transfer process of the C2RMF algorithm and illustrates the corresponding relationship between the contents of the CSV and RDF files.

Semantic Web Query Process.
To retrieve transportation route alternatives from the Solid server, semantic web queries were developed using SPARQL. SPARQL is the W3C standard for creating, querying, and updating linked databases. SPARQL is the standard query language for the RDF model. SPARQL has proven to be a powerful querying language. SPARQL queries in Solid are divided into two queries: local queries and link-following queries. e local query can access only predicates that are located on the local user's pod, whereas the link-following query can access predicates on many pods [17]. e query process retrieves transportation route alternatives in terms of routeId or tripId.
e semantic web query considers the synonym heterogeneity for querying between two stations because the station names are diverse in the domain ontology with the same separate definitions.
Similar to SQL, SPARQL retrieves data from the query dataset through a Select statement to determine which result of the selected data will be returned. Additionally, SPARQL uses a Where clause to state criteria to discover a match in the query dataset. A SPARQL query comprises five parts: namespace prefix, result set, dataset, query triple pattern, and modifiers. Similar to SQL, Running a SPARQL query will return all the match data. In the SPARQL query example    Display the tra c jam of searching routes User 5

Journal of Advanced Transportation
Suggest an appropriate route based on user needs System 6 Search car parking based parking date and parking close to where user needs it through users' position User 7 Create account and sign in, and nd an appropriate route based on user needs and book it User 8 Save favorite routes and recent searching routes User 9 Help and FAQ User 10 Social media including Twitter, Instagram, and Facebook User 11 Suggest an appropriate route to go to car parking System 12 e system can restrict di erent levels of users so only authorized users can do what they ought to do System  below, the SELECT clause specifies the result variable to be returned, and * means to return all result variables. e FROM clause defines the dataset to query. WHERE clause specifies query conditions. Finally, the query returns all these declared query variables in the final subset, in terms of the subjects, predicates, or objects they are defined to in the bus.ttl. "Order by? TripHeadsign" orders the subset in alphabetical order. e SPARQL query is shown in Figure 5.

Visualization.
ere are two key modules during the visualization component in the web-based data-sharing system visualization. Google Maps can establish a stable connection from the server to the client and provide downloading of extra map information for displaying map information on the client [33]. Additionally, the application programming interface (API) function, provided by Google, comprises a couple of classes, functions, and data structures that could be used by a developer through JavaScript or others [34]. In this system, the Google Map API is called to initialize the selected map area for display on the web page.
SPARQL is used to retrieve relative route data through the pod server, and then CSS and JavaScript are used to show selected Uber and bus routes, displaying the message on the map based on retrieving data. Figure 6 shows the visualization framework for implementing a web-based datasharing system.

Experiment
e experiment can be dived into two parts, the performance test of the C2RMF algorithm and system testing.

Conversion Performance Test.
To verify the performance of the C2RMF algorithm, two ways of converting CSV to RDF data were tested in this testing process, one is the C2RMF algorithm, and the other is a transformer tool named stlab.csv2rdf-1. is csv2rdf is a typical Java-based and open-source tool, which depends on Apache Jena to convert CSV data into RDF [35]. ere are other transfer tools that can achieve this purpose, such as Geometry RDF and Table 6. But Csv2rdf is chosen as it is very easy and intuitive to use, and well provide to achieve data transformation.

Testing Environment.
e system hardware testing environment includes a Huawei Cloud Server configured with Kunpeng 920 2.6 GHz, 8vCPUs, and 32 GB of RAM.
e system software testing environment comprises an Open Euler 20.03 64 bit operating system (server), JDK 1.8.0.

Testing Data.
Four datasets from different sources were used for this test [36].

Testing Results.
ere are two steps in the conversion performance test process, one step is the comparison of the       is part of system testing mainly includes functional testing and non-functional testing.

Testing Environment.
e system hardware testing environment includes a desktop computer con gured with an Intel (R) Core(TM) i7-4702MQ CPU 2.2 GHz and 8 GB of RAM. e system software testing environment comprises a Windows 10 Professional 64 byte operating system, a local Solid server version 5.1.6, SpringBoot version 2.1.6, an embedded Tomcat server, which is considered a local web server, and a transformer tool named CSV Transformers RDF algorithm.

Testing Data.
During the test process, diverse CSV data were collected from di erent data sources using diverse addressing mechanisms [24][25][26].

Functional Testing and Results
. Functional testing was used for an achieved program. e aim was to demonstrate that it supplies all of the behaviors required of it. e option of test cases is in terms of the user requirement of the software entity under test [37]. Functional testing is a form of black-box testing. Based on the above, functional testing was considered the main testing technology during the testing phase. ese gures show the functional testing results based on the testing data. Figure 9 shows the bus route list for sharing data on the Solid platform. Figure 10 shows a bus route on the Google Maps and retrieves data from the Solid platform. Figure 11 shows an Uber route on the Google Maps and retrieves data from the Solid platform. Figure 12 shows a car parking position on the Google Maps and retrieves data from the Solid platform.
Based on Table 7, the functional testing results of the new system are listed in Table 8. Table 4, the non-functional testing results are listed below:     Usability Testing. ere are many guidelines and criteria that were built during usability testing, but the developer could not entirely depend on these guidelines [38]. When following these guidelines and criteria and during the development of the website, it is necessary for users to easily use the website [39]. In terms of the website completed in this paper, it is easy for users to visit and use. erefore, the website meets the usability requirements. Security Testing. e most necessary criterion for a web application is probably security. is involves regulating the retrieval of data, certifying user authorities, and storing and protecting system data. Compatibility Testing. e compatibility of the website is a crucial aspect. e website has better compatibility with various available main browsers, including Google Chrome, Firefox, Safari, and Internet Explorer. Additionally, the website can be visited by the Linux and Windows operating systems, which shows that the website also has operating system compatibility. Performance Testing. e entire web page loads in less than 5 s in the testing environment. However, as the Solid server and web server were built on the local computer, the test performance in the testing environment is uncertain. erefore, performance testing needs to be further verified on a remote server. Extensibility Testing. As the RDF data model was used on the website, it is easy to integrate other new public transportation data, which demonstrates that the website provides good extensibility. Availability Testing. According to the test local Solid server, it is obvious that the website supplies good availability through the Solid platform.

Non-Functional Testing and Results. According to
6.3. Experiment Summary. In total, approximately 80 typical test cases were executed and presented in this paper. Each test case is designed to produce useful data. ese test cases were chosen to develop data conversion, data storage, data retrieval, and visualization. Additionally, it is necessary to verify the user requirements and non-functional requirements of these test cases. In this section, some typical examples of these tests are presented, such as converting a heterogeneous data source into uniform RDF data, storing RDF data in the pod server to retrieve data through SPARQL, and finally displaying data on the web page based on Google Maps, CSS, and JavaScript. e test results demonstrate that the new website can meet user requirements.
Furthermore, applications are implemented as clientside web, which reads and writes data directly from the pods in this website in terms of a Solid platform. Multiple applications can also reuse the same data on pod servers. In addition, the new website system sustained the view that sharing public transportation data services could be improved through semantic web and Solid technologies. e website was also developed that is free and provides several protocols, for example, SPARQL. It is efficient for developing applications through Solid platforms.
In addition, the new system is a well-established standard for publishing and managing unstructured or structured data on the web, gathering and bridging knowledge from different data sources. C2RMF algorithm was developed to convert CSV into RDF, and also its performance test was verified. Figure 7 shows the comparison of the execution time of the C2RMF algorithm and the csv2rdf on different datasets. Figure 8 shows the comparison of the execution time of the C2RMF algorithm and the csv2rdf based on the different number of rows. In terms of Figures 7  and 8 Figures 6 and 7, and these results demonstrate the effectiveness of the C2RMF algorithm compared with the traditional method because our method optimizes the processing flow and efficiency during the process of converting CSV files to RDF files. erefore, the C2RMF algorithm can not only convert the original CSV data into RDF data without changing the data but also has high integration and extensibility.

Experiment Analysis.
On the other hand, Figure 9 shows the alternative bus route list to arrive at the destination based on this CSV file of the National bus system, which aims to provide the best alternative routes to arrive at the destination in urban areas according to open CSV data. Figure 10 shows a further test case; according to Figure 9, it retrieved each bus station of the selected bus route through SPARQL and finally displayed them on the website through the Solid platform. Figure 11 shows a test case from the CSV file of the Uber system. It suggests that these alternative routes arrive at the destination. e processing flow is similar to that in Figure 10, and the route is also shown on the website with Google Maps. Figure 12 shows the test case from a car parking CSV file, which suggests suitable car parking locations for a car based on its location and is shown on the web page with Google Maps.
However, there are some limitations: (i) e data source is a single type file: among the different data sources, only the CSV file was selected and converted into an FDF file through our developed converting algorithm during the testing process. In a real environment, other data source files must be selected and converted. (ii) History data source: the different open data sources were history data and not real-time data. It is helpful to provide online data in a real environment. (iii) Lack of SPARQL interface: website developers need to be experienced with diverse data schemas and query evaluations to solve effective SPARQL queries.

Conclusion and Recommendations
In this paper, we proposed a novel approach that mainly included the proposed data processing method, the new C2RMF algorithm, the proposed system framework, and the web-based data-sharing system. e approach can achieve to integrate and share heterogeneous public transportation data to provide personalized sharing of these data between different users. e research results on the publicly available datasets demonstrate the significance of the approach from two aspects. (1) e proposed data processing method involves the management process of the full life cycle of data including data collected, data integrated and generated, data stored, data searched, and data shared. (2) e proposed approach provided a unified data operation mode for information storage and retrieval of heterogeneous public transportation data. It is useful to manage these data. In future work, it would be very interesting to do the following. (1) We plan to convert other data types of public transportation including structured data into RDF data through our C2RMF algorithm. (2) We will try to forecast short-term traffic flow based on these public transportation data.

Data Availability
No data were used to support this study.

Conflicts of Interest
e authors declare that they have no conflicts of interest.