Traditional data sources are not sufficient for measuring the United Nations Sustainable Development Goals. New and nontraditional sources of data are required. Citizen science is an emerging example of a non-traditional data source that is already making a contribution. In this Perspective, we present a roadmap that outlines how citizen science can be integrated into the formal Sustainable Development Goals reporting mechanisms. Success will require leadership from the United Nations, innovation from National Statistical Offices and focus from the citizen-science community to identify the indicators for which citizen science can make a real contribution.

Building on abstract reference models, the Open Geospatial Consortium (OGC) has established standards for storing, discovering, and processing geographical information. These standards act as a basis for the implementation of specific services and Spatial Data Infrastructures (SDI). Research on geo-semantics plays an increasing role to support complex queries and retrieval across heterogeneous information sources, as well as for service orchestration, semantic translation, and on-the-fly integration. So far, this research targets individual solutions or focuses on the Semantic Web, leaving the integration into SDI aside. What is missing is a shared and transparent Semantic Enablement Layer for SDI which also integrates reasoning services known from the Semantic Web. Instead of developing new semantically enabled services from scratch, we propose to create profiles of existing services that implement a transparent mapping between the OGC and the Semantic Web world. Finally, we point out how to combine SDI with linked data.

Digital Earth (DE) is a powerful metaphor for the organisation and access to digital information through a multi-scale three-dimensional representation of the globe. Recent progress gave a concrete body to this vision. However, this body is not yet self-aware: further integration of the temporal and voluntary dimension is needed to better portray the event-based nature of our world. We thus aim to extend DE vision with a Nervous System in order to provide decision makers with improved alerting mechanisms. Practical applications are foreseen for crisis management, where up-to-date situational awareness is needed. While it is traditionally built through trusted sources, citizens can play a complementary role by providing geo-referenced information known as Volunteered Geographic Information (VGI). Although workflows have been implemented to create, validate and distribute VGI datasets for various thematic domains, its exploitation in real time and its integration into existing concepts of DE, such as spatial data infrastructures, still needs to be further addressed. In this paper we suggest to bridge this gap through Sensor Web Enablement for VGI, where VGI sensing becomes a sense of the DE's Nervous System. We discuss this approach and its applicability in the context of a forest fire scenario.

, Como, Italy. The authors were invited to submit an extended version to this special issue. Apart from editorial changes, we included a new section on the sensing of multiple VGI platforms and an extensive discussion section on generalizing the approach towards a nervous system for the next generation of geospatial information infrastructures (Digital Earth). Pointers to related works have been extended, too.Abstract. There is a long tradition of non specialists contributing to the collection of geo-referenced information. Thanks to recent convergence of greater access to broadband connections, the availability of Global Positioning Systems in small packages at affordable prices, and more participative forms of interaction on the Web (Web 2.0) vast numbers of individuals became able to create and share Volunteered Geographic Information (VGI). The potential of up to 6 billion persons to monitor the state of the environment, validate global models with local knowledge, contribute to crisis situations awareness, and provide information that only humans can capture is vast and has yet to be fully exploited. Integrating VGI into Spatial Data Infrastructures (SDI) is a major challenge, as it is often regarded as insufficiently structured, documented or validated according to scientific standards. Early instances of SDIs used to have limited ability to manage and process geosensor-based data (beyond remotely sensed imagery snapshots), which tend to arrive in continuous streams of real-time information. The current works on standards for Sensor Web Enablement (SWE) aim to fill this gap. This paper shows how such standards can be applied to VGI, thus converting it in a timely, cost-effective and valuable source of information for SDIs. By doing so, we extend previous efforts describing a workflow for VGI integration into SDI and further advance an initial set of VGI Sensing and event detection techniques. Examples of how such VGI Sensing techniques can support crisis information system are provided. The presented approach serves central building blocks for a Digital Earth's nervous system, which is required to develop the next generation of (geospatial) information infrastructures.

The vision of a Digital Earth calls for more dynamic information systems, new sources of information, and stronger capabilities for their integration. Sensor networks have been identified as a major information source for the Digital Earth, while Semantic Web technologies have been proposed to facilitate integration.. So far, sensor data is stored and published using the Observations & Measurements standard of the Open Geospatial Consortium (OGC) as data model. With the advent of Volunteered Geographic Information and the Semantic Sensor Web, work on an ontological model gained importance within Sensor Web Enablement (SWE). In contrast to data models, an ontological approach abstracts from implementation details by focusing on modeling the physical world from the perspective of a particular domain. Ontologies restrict the interpretation of vocabularies towards their intended meaning. The ongoing paradigm shift to Linked Sensor Data complements this attempt. Two questions have to be addressed: (i) how to refer to changing and frequently updated data sets using Uniform Resource Identifiers, and (ii) how to establish meaningful links between those data sets, i.e., observations, sensors, features of interest, and observed properties? In this paper, we present a Linked Data model and a RESTful proxy for OGC's Sensor Observation Service to improve integration and inter-linkage of observation data for the Digital Earth.Keywords: Digital Earth, Geospatial data integration, Spatial Data Infrastructure, Earth Observation, Data exchange models, Linked Data, Semantic Enablement * Corresponding author: Krzysztof Janowicz; email: jano@geog.ucsb.edu 1 MotivationThe initial vision of a Digital Earth was first formulated by former US Vice President Al Gore as a multi-resolution, three-dimensional representation of the planet, into which we can embed vast quantities of geo-referenced data (Gore, 1998). Ten years after this speech, Craglia et al. published a position paper to argue that this vision has not yet been achieved (Craglia et al., 2008). In parallel to the growing availability of information, the need to better understand the interplay of environmental and social phenomena has also increased, thus requiring more dynamic systems, new sources of information, and stronger capacities for their integration. The Sensor Web has been identified as a central building block to address these challenges (De Longueville et al., 2010). A digital nervous system for the globe has been suggested as a vibrant approach for the Digital Earth. An implementation based on Spatial Data Infrastructures (SDI), especially on the Sensor Web Enablement (SWE) standards of the Open Geospatial Consortium (OGC), has been proposed. These infrastructures do not only deliver data but also offer geospatial processing capabilities and the final rendering on a virtual globe. Grounded in spatial and temporal reference systems, the outcomes of a variety of services can be combined into a multi-layered representation of the Earth's surface and help to an...