Design and implementation of an on-demand feature extraction web service to facilitate development of spatial data infrastructures
Introduction
The development of spatial data infrastructure (SDI) has evolved as a central driving force in the management of spatial information over the last decade (Williamson, Grant, & Rajabifard, 2005). More than half the world’s countries claim that they are involved in some form of SDI development (Crompvoets, Rajabifard, Bregt, & Williamson, 2004). SDI concepts and models have also been recently used in different applications, such as disaster management (Mansourian, Rajabifard, Valadan Zoje, & Williamson, 2006), natural resource management, and wireless applications (Davies, 2003). With such wide range of activities, Masser (2005a) used the term ‘SDI phenomenon’ to describe the events that have taken place in this field over the last 10–15 years.
SDI is an initiative that intends to create an environment that will enable a wide variety of users to access, retrieve, and disseminate spatial data in an easy and secure way. In principle, SDIs allow the sharing of data; this is extremely useful, because it enables users like urban and environment managers to save resources, time, and energy when trying to acquire new datasets by avoiding duplication of expenses associated with the generation, maintenance, and integration of data. SDI aims to establish the relationship between people and data through appropriate policy-making, standardization activities, and the creation of accessing networks (the general SDI model: Rajabifard, Feeney, & Williamson, 2002). SDI is also an integrated, multi-level hierarchy of interconnected SDIs based on collaboration and partnerships among different stakeholders (SDI hierarchy: Rajabifard, Feeney, & Williamson, 2003).
Development of SDI is a matter of different technical, technological, social, institutional, and economical challenges (de Man, 2006, Mansourian et al., 2006, Masser, 2005b, Masser et al., 2007, Williamson et al., 2003). One of the important problems relates to the gap in spatial data (INSPIRE, 2003, Karatunga, 2002, Omran et al., 2006). The ‘gap’ in spatial data relates to unavailability of required data and/or the unreliability of available data due to low accuracy, being out-of-date, incompleteness, and similar issues. This problem is more critical in urban environments because of their fast development and changes. Since current manual methods for the production of spatial data is time-consuming and expensive, this issue is considered as an important problem for the development of SDI at its initial stages.
Remote sensing is a multidisciplinary science and technology that can collect data from any area in a short period of time. It offers a good solution that fulfills the data requirements of SDIs. Remote sensing offers data as an image in raster format, however, and most of the GIS (geographical information system) users would like to have spatial data at the feature level in a vector format. Automatic/semi-automatic feature extraction techniques from remote sensing data allow feature level data to be obtained with lower costs and shorter times than manual/traditional techniques.
This paper suggests the establishment of on-line Web services that provide users with data using an on-demand data production approach based on automatic/semi-automatic feature extraction techniques from satellite images. Web services equipped with automated/semi-automated feature extraction engines can thus be established. If the datasets required by a user are not available, then the user can connect to the Web service through the Internet, introduce the extent of the region of interest, and enter the required data layers to the engine. The system chooses the appropriate available satellite images from archive. Datasets that the user needs are produced automatically using advanced information processing techniques. The produced data can be added to the list of a spatial clearinghouse or catalogue service in order to make the data searchable and accessible for other users in the future.
Different challenges for making such a service operational and usable exist. From a SDI perspective, one technical challenge relates to spatial interoperability. Based on the OGC1 Reference Model,2 spatial interoperability refers to the “capability to communicate, execute programs, or transfer spatial data among various functional units in a manner that requires the user to have little or no knowledge of the unique characteristics of those units”. As this definition suggests the non-interoperability of spatial processing systems hampers the sharing of spatial data and services among software applications. In this context, two kinds of non-interoperability – data and access non-interoperability – can be identified3 (Peng, 2004).
Data non-interoperability implies that different spatial processing systems use internal data formats and produce data in formats that are different and in most cases proprietary. As a result, data sharing among different systems in a SDI environment is difficult. Access non-interoperability means that different spatial processing systems use proprietary software access methods with proprietary software interfaces, which restrict inter-process communication among various spatial processing systems. In other words, interface definition languages, communication protocols, communication ports, and even object transfer mechanisms vary in each software development platform. The software platform, which is used to develop the spatial processing system, thus imposes the use of specific and proprietary communication methods among various parts of the system. For this reason, different spatial processing systems developed by different software development platforms cannot communicate and share services automatically in an interoperable manner. This paper suggests an integrated utilization of Web service technologies (from the IT world) as well as OGC’s specifications and encodings (from the spatial information community) to resolve the non-interoperability problem for the proposed Web service.
In the context of this research, the development of a Web service that uses automatic feature extraction techniques to produce data for users in a SDI environment is ongoing. OGC frameworks and Web service technologies are used for the development of the system to provide spatial interoperability between the system and users. This paper describes the results of the project, including the development of a prototype system.
It is notable that current studies indicate that the deployment of spatial Web services is known as a significant factor facilitating the development of SDIs (Najar, Rajabifard, Williamson, & Giger, 2007). Web services can support the user in processing, accessing, and visualizing data. Most current activities developing spatial Web services and relevant standards focus on access (e.g. catalogue services) and visualization (e.g. Web Map Services), whereas less attention has been paid to processing aspects in the SDI environment. This research also addresses the benefits that can be gained from spatial Web services in the context of processing tasks within a SDI environment.
Section snippets
Methodology and framework
As highlighted earlier, automatic feature extraction techniques, OGC frameworks, and Web service technologies are three main components required in the development of a Web-based service that (1) has the capability of on-demand data production from remote sensing data and (2) acts as a step forward to facilitate SDI development. This section describes the algorithm, methodologies, and frameworks utilized for development of the prototype system.
Development of a prototype service: a case study
A prototype system is developed as part of this research in order to investigate and demonstrate the proposed Web service, which is an on-demand data production system from satellite images. The system is developed based on Web client–server architecture. An automatic road extraction engine, raster-to-vector conversion engine, GML data production engine, WMS, and database construct the major components of the system at the server’s side (Fig. 2). Proper user interfaces at the client’s side are
Conclusions and future trends
The gap in spatial data is one of the major issues for developing SDIs. This paper proposes a Web-based, on-demand data production service on the Internet that uses feature extraction techniques from satellite images as a solution to this problem.
In order to investigate this suggestion, a prototype service was developed. OGC frameworks and Web service technologies were utilized for the development of the system; using such standards, data and access interoperability were achieved. The results
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