A model driven approach for the development of metadata editors, applicability to the annotation of geographic information resources

Abstract

Metadata are a key element for the development of information infrastructures because they facilitate the semantic description of contents and services. However, the diversity and heterogeneity of metadata standards have become a barrier for the generation of these metadata. Many metadata editors are not useful anymore because they do not support the latest version of metadata standards or the new profiles arisen in the market. Thus, this work proposes a model driven approach for the development of metadata editors, more focused on the generic treatment of metadata models than on the development of specific edition forms for a reduced set of metadata standards. This approach has been tested in the context of Spatial Data Infrastructures for the development of an Open Source tool called CatMDEdit. Additionally, the approach could be also applied to improve the efficiency of any metadata editor using a metamodeling development strategy.

Introduction

Spatial Data Infrastructures (SDI) are special types of information infrastructures consisting of the relevant base collection of technologies, policies and institutional arrangements that facilitate the availability of and access to spatial data. Traditionally, spatial data (also known as geographic information) were the core component of Geographic Information Systems (GIS), which is the term commonly used to refer to the software packages that allow one to capture, store, check, integrate, manipulate, analyze and display them. However, the potential of spatial data as an instrument to facilitate decision-making and resource management in diverse areas (e.g., natural resources, facilities, cadaster or agriculture) of government or private sectors has led to the evolution of GIS into the broader concept of SDI [1].

One of the main elements for the success in the development of SDI or any other type of information infrastructure is the appropriate annotation of resources to be accessed and distributed by means of metadata. Metadata constitute the mechanism to characterize data and services (e.g., descriptions of the content, quality, condition, authorship and any other features) in order to enable other users and applications to make use of such data and services. However, due to the heterogeneity of contents in information infrastructures, it is not possible to consider a unique metadata model or schema.

The diversity of metadata standards has been a critical issue for the development of SDIs. During the last fifteen years, standardization bodies have proposed different metadata standards such as the Content Standard for Digital Geospatial Metadata (CSDGM) [2] or ISO 19115 Geographic Information — Metadata [3]. Additionally, apart from the standards, it is also common to find application profiles and extensions of these standards. The Dublin Core Metadata Initiative [4] defines a Metadata Application Profile as a declaration of the elements (either selected from the standard, or new elements) that an organization or user community employs in their metadata, and how these elements have been customized to a particular application domain. Within the SDI context, there are multiple examples of metadata profiles for the description of remote sensing data [5], [6], environmental data [7], [8], or the customization of general metadata standards such as Dublin Core [9].

In parallel to the definition of this wide range of geographic metadata standards, there has been an increasing need for desktop or web-based metadata editors able to manage these complex standards (hundreds of elements with different data types organized in hierarchical entities) and providing, among other functionalities, an internationalized interface, online help, validation of standard conformance, and serialization to XML or other semi-structured formats. Through different surveys [10], [47], it can be verified that the development of these metadata editors has been highly promoted during the last years: about 50 tools have been published by different organizations and software companies. At the beginning, these tools were developed to support a unique metadata model. However, these first tools based on fixed structures became rapidly out-of-date. In contrast, nowadays software developers are sensitive to this problem of flexibility. Instead of implementing a particular metadata model, most of the current editors enable as well the configuration of the model using some kind of schema language.

From a software engineering perspective, we could say that annotation tools in the SDI context have moved towards a higher level of abstraction: the focus is now on the management of metamodels. According to ISO/IEC 11179‐3 [11], a metamodel provides a mechanism for understanding the precise structure and components of the specified models, which are needed for the successful sharing of the models by users and/or software facilities. Although the available documentation of most editors does not reveal a conscious interest on metamodels, the key component of these tools is the mechanism to describe the different metadata standards that can be used later to customize dynamically the software for editing metadata in conformance to these standards.

The need for flexibility and the consequent increase of model abstraction is leading the development of metadata editors to a stage that, although not directly intended, is close to the software engineering paradigm of Model Driven Engineering (MDE). MDE focuses on models as the primary artifact in the development process, with transformations as the primary operation on models, used to map information from one model to another [12]. However, in order to avoid ad-hoc and un-efficient implementations of this MDE paradigm, it is important to follow a systematic and acknowledged methodology such as the one defined by the Model Driven Architecture (MDA), the OMG instantiation of MDE [13], [19]. Although in the SDI context MDA has been considered for interoperability issues and data access services [14], [15], [16], it has received little attention for the development of metadata editors. The objective of this work is to provide the guidelines and framework to accomplish the development of metadata editors according to MDA in order to facilitate their rapid development, decreasing the development effort and, at the same time, augmenting their efficiency and flexibility. This paper presents the different domain specific languages and transformations required to define and transform the models involved in the development of a metadata editor: the Platform Independent Model (PIM) for the representation of supported metadata schemas; the Platform Specific Models (PSM) for the specification of a set of GUI (Graphical User Interface) edition forms, and the representation of controlled vocabularies; and the transformation of PSM models into code.

The rest of this paper is structured as follows. Section 2 presents our proposal to apply an MDA approach in the development of metadata editors. Following this proposal, Section 3 describes how this proposal has been used in CatMDEDit, an open-source metadata editor, for the support of different metadata standards and profiles. Then, Section 4 describes the related work in the development of annotation tools from a metamodeling perspective, and discusses the benefits from applying an MDA development approach. Finally, Section 5 draws some conclusions and outlines future work.