## 1 Introduction: From Heterogeneous Sensors to the Sensor Web  
A _sensor_ is defined from an engineering point of view as a device that converts a physical, chemical, or biological parameter into an electrical signal [1]. Common examples include sensors for measuring temperature (_i.e._, a thermometer), wind speed (an anemometer) conductivity, or solar radiation. While a sensor is the most basic unit, a _sensor system_ is an aggregation of sensors, attached to a single platform [2]. Examples are a weather station with attached sensors, or a combination of heart frequency and blood pressure sensors carried by a human or animal. A sensor or a sensor system may be abstracted as a _sensor resource_. A _sensor network_ consists of a number of spatially distributed and communicating sensor resources [3]. Sensor technology is continuously improving as the devices become smaller, cheaper, more intelligent, and more power efficient. In consequence, more and more application fields are making use of these technologies. Examples are disaster management, environmental monitoring, precision agriculture, early warning systems, home as well as public security, or human health [4-6]. The kinds of sensor resources utilized in these applications may be stationary or in motion and could gather data in an _in-situ_ or remote manner. Due to the large variety of sensor protocols and sensor interfaces, most applications are still integrating sensor resources through proprietary mechanisms, instead of building upon a well-defined and established integration layer. This manual bridging between sensor resources and applications leads to extensive adaption effort, and is a key cost factor in large-scale deployment scenarios [7]. This issue has been the driving force for the Open Geospatial Consortium (OGC) to start the Sensor Web Enablement (SWE) initiative (_[[http://www.ogcnetwork.net/swe_](http://www.ogcnetwork.net/swe_)]([http://www.ogcnetwork.net/swe_](http://www.ogcnetwork.net/swe_))) back in 2003. Within the SWE working group a suite of standards has been developed which can be used as building blocks for a _Sensor Web_. SWE defines the term Sensor Web as \"Web accessible sensor networks and archived sensor data that can be discovered and accessed using standard protocols and application programming interfaces\" [8]. First described by [PERSON] _et al._ in 1999 [9], a _Sensor Web_ was considered as an autonomously organized wireless sensor network which can be deployed to monitor environments. As a smart macro instrument for coordinated sensing [10], [PERSON]'s Sensor Web concept consists of sensor nodes which not only collect data, but also share their data and adjust their behaviour based on that data. Thereby, the term \"Web\" within [PERSON]'s \"Sensor Web\" relates to the intelligent coordination of the network rather than the World Wide Web (WWW) [11]. Later, the meaning of \"Sensor Web\" changed and it was more and more seen as an additional layer integrating sensor networks with the WWW and applications [12-14]. Today, the notion of \"Sensor Web\" has been largely influenced by the developments of the SWE initiative. It is defined as an infrastructure which enables an interoperable usage of sensor resources by enabling their _discovery_, _access_, _tasking_, as well as _eventing_and _alerting_ within the Sensor Web in a standardized way. Thus, the Sensor Web is to sensor resources what the WWW is to general information sources--an infrastructure allowing users to easily share their sensor resources in a well-defined way [15]. It hides the underlying layers, the network communication details, and heterogeneous sensor hardware, from the applications built on top of it. To achieve this, SWE incorporates models for describing sensor resources and sensor observations. Further, it defines web service interfaces leveraging the models and encodings to allow accessing sensor data, tasking of sensors, and alerting based on gathered sensor observations.