The Full Picture

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An operational geospatial IT solution spanning multiple

data sources and disciplines, such as GEOSS, is needed to

bridge the ‘isolated’ interoperability building blocks of

Sensor Web, discovery and exploitation services. Thus,

GEOSS will be key to the successful path of the Sensor

Web and vice versa. GEOSS depends on a powerful sensor

data infrastructure to obtain its major tasks, while provid-

ing several key components necessary to make the Sensor

Web fully operational. The GEOSSWeb portal implements

a single point of entry either directly to the Sensor Web

services and products, or via its linked community portals.

In conjunction with the GEOSS clearinghouse, the GEOSS

registry and community catalogue services, Sensor Web

components will be explored for reuse in new applications

built on top of the existing Sensor Web infrastructure. The

following example illustrates how GEOSS empowers the

set-up of new applications based on sensor data and made

available by Sensor Web components.

The Advanced Fire Information System (AFIS) version

two, is one of the early examples of a GEOSS application

based on Sensor Web technology. Currently in an experi-

mental phase, it integrates a growing number of data

sources and sensor tasking components. Once completed,

AFIS will be the ideal example of an application that makes

use of a number of systems that were united in the GEOSS.

AFIS analyses veld fires in sub-Saharan Africa.

Moderate-resolution imaging spectroradiometer satellite

data from Aqua and Terra satellites is continuously parsed

for hot pixels, which indicate potential veld fires. This

hotspot data is provided by a sensor observation service

instance located in South Africa. Hotspots located close

to power lines, commercial forestations or urban areas

trigger NASA’s EO1 satellite automatically. The tasking

uses a sensor planning service to upload the observation

requests to the satellite. The Hyerperion and ALI sensors

onboard EO1 produce vast amounts of data that is down-

linked and further processed in the US, before it is sent to

a data store located in Canada. This data store is façaded

by a Web coverage service interface. Standardized by the

OGC, the Web coverage service allows clients compliant

to the standard to retrieve the data automatically.

The client application integrates additional data sets

provided by sensor observation services, including current

wind situation and topographical data sets, to calculate

the spread of the fires. The client application sends alerts

via the Web notification service in case the fires threaten

valuable areas. All AFIS components can be explored in

the GEOSS clearinghouse. Component interfaces are regis-

tered at the GEOSS registry. Since each is fully standards

compliant, it can be used within other applications.

The Sensor Web, as a new Earth observation system,

opens up a new avenue to fast assimilation of data from

various sensors (both in situ and remote), as well as to

accurate analysis and informed decision making. It

provides the foundation layer for GEOSS, in the form

of raw and processed sensor data that is the basis for the

higher level information in Earth observations essential

to understanding the current and future situation of

planet.

Sensor Web is a meta-platform that integrates arbitrary sensors and

sensor networks, each maintained and operated by individual institu-

tions. Examples include the Australian Water Resources Network, the

European Environment Information and Observation Network, and

the South African Earth Observation Network. This reflects the exist-

ing legal, organizational and technical situation. Sensors and sensor

systems are operated by various organizations with varying access

constraints, as well as security, data quality and performance require-

ments. The architecture of the Sensor Web allows the integration of

individual sensors and of complete sensor systems without the need

for fundamental changes to the legacy systems.

Once connected to the Sensor Web, data sets may be used multiple

times in applications never intended by the original system set-up.

Traffic sensors initially deployed to avoid traffic jams by means of

dynamic traffic control might be used to calculate the carbon dioxide

ratios of highway sections in another application. Satellites with differ-

ent sensors on board might be used in a variety of application domains

that were not primarily targeted, simply because the interoperable inter-

faces allow users to task the satellite based on distinct requirements.

Consumers use the Internet to access distributed data sources by

invoking Web services. The Web services interfaces must be standard-

ized to achieve interoperability between all data, or sensor providers

and consumers. Currently, the most prominent approach to standard-

izing Sensor Web interfaces is the Sensor Web Enablement Initiative

run by the Open Geospatial Consortium (OGC), a not-for-profit, inter-

national, voluntary consensus standards organization that leads the

development of standards for geospatial and location-based services.

The consortium unites more than 350 software vendors, research

institutes and government agencies worldwide. In the context of the

Sensor Web Enablement Initiative, a number of standards have been

developed during the past seven years to provide a sound technologi-

cal foundation on which to build the Sensor Web. The standards cover

a number of requirements. First, sensors and sensor data have to be

discoverable. Queries based on observed phenomena, temporal reso-

lution, spatial extent, and quality levels, among other criteria, allow

the filtering of the vast amounts of available data sources. The infor-

mation obtained has to be understandable and processable by machines.

This requires a high level of syntactical and semantic expressiveness.

Sensors will be tasked to the specific needs of various user groups and

will send alerts when a sensor measures a particular phenomenon.

Overall, the OGC has published seven standards: four Web service

interface specifications and three data encoding and metadata

languages. The sensor observation service retrieves sensor data, the

sensor planning service tasks sensors and models, the sensor alert

service pushes real-time information to the registered user, and the Web

notification service delivers sensor information by means of various

communication protocols. The sensor model language describes

sensors, platforms and sensor data processing chains. Abstract models

and XML-based implementations to encode sensor data are provided

by the observation and measurement. The transducer mark-up language

supports description and encoding of continuous data streams.

Although technical solutions for building the Sensor Web are avail-

able, there is a lack of consolidated effort to make all data sets

discoverable and accessible. This is mainly due to the fact that the

Sensor Web specifications enable access to sensors and observation

data, but do not provide any catalogues, dictionaries or registries that

facilitate discovery and exploitation. This was done intentionally. Other

internationally adopted standards, provided by the International

Organization for Standardization and the OGC, cover this field.

GEOSS C

OMPONENTS

– O

BSERVING

YSTEMS