Digital Earth was the label given to a visionary concept, made popular in 1998 by former US vice president Al Gore, for describing a virtual representation of the Earth on the Internet that is spatially referenced and interconnected with the world’s digital knowledge archives.
In a speech prepared for the California Science Center in Los Angeles on January 31, 1998, Gore articulated a digital future where a young girl could sit before a computer generated 3-dimensional spinning Earth and access information from around the planet with vast amounts of scientific, natural, and cultural information to describe, entertain, and understand the Earth and its human activities. This vision states that any citizen of the planet, linked through the Internet, should be able to access vast amounts of free information in this virtual world, however, a vast commercial marketplace of products and services was envisioned to co-exist.
Digital Earth continues to evolve along two distinct lines of organization constructs. One construct is through a growing and deliberate global partnership of Non-Governmental Organizations, educators, business, and government leaders collaborating together with the goal of enable future generations unprecedented technical and educational facilities for exploring the Earth, better understanding its systems, and investigating impacts of human activities. This Digital Earth community has dedicated itself to building a global commons promoting down-to-Earth solutions based on cooperative use of standards, databases, and tools. Four international symposia (see International Symposium on Digital Earth) have been held around the world representing this community, with the 5th International Symposium on Digital Earth scheduled to be held in San Francisco during June 2007. The second line of Digital Earth organizational manifestation is the business sector leaders, such as Google, Yahoo, and MSN offering handy “digital earth” applications to anyone interested in creating customized mapping applications; applications led by travel, real estate, tourism, and business location drivers. The recent Where 2.0 conference inaugurated in 2005 represents a prime example of the new commercial focus that caters specifically to the hackers and developers using map mash-ups and web-mapping applications linked to the large corporate web-based spatial search engines. Through daily weather forecasts, Google Maps, and nightly news coverage, citizens are rapidly becoming accustomed to the Digital Earth interface.
Digital Earth Background
United States of America
Technology developments that support the current Digital Earth technological framework can be traced to U.S. computing advances derived from the Cold War competition, the space race, and commercial innovations. Therefore, many innovations can be tracked to corporations working for the Department of Defense or NASA. However, the philosophical foundations for Digital Earth can be more closely aligned with the increased awareness of global changes and the need to better understand the concepts of sustainability for the planet’s survival. These philosophical roots can be traced back to visionaries, such as Buckminster Fuller who suggested half a century ago that we need to create a GeoScope, analogous to a microscope to examine and improve our understanding of the planet Earth.
In 1999, NASA was selected to head the Interagency Digital Earth Working Group (IDEWG) due in part to its stellar reputation for technology innovations and in part for the agency’s focus on the study of planetary change. The new initiative was located in the NASA’s Office of Earth Sciences. This titular focus was considered necessary to help align over 17 government agencies and keep sustainability and Earth oriented applications as a guiding principle for the Digital Earth enterprise. Components for development of 3-D Earth graphic-user-interfaces (GUIs) were placed into various technological sectors to stimulate cooperative development support; including education, museums, research and development. While initially limited to government personnel, industry and academia were early observers attending IDEWG workshops to discuss different topics such as, visualization, information fusion, standards and interoperability, advanced computational algorithms, digital libraries, museums, et cetera. In March 2000, at a special meeting hosted by Oracle Corporation in Herndon, Virginia, industry representatives showcased for the IDEWG over a dozen enterprising technologies demonstrated the range of promising 3-D visualization prototypes. Within two years, these prototypes were captivating international audiences, including Kofi Annan and Colin Powell, in government, business, science, and mass media who began to purchase the early commercial geobrowsers. Just as the spectacular Apollo photography of Earthrise provided an inspiring Earth-centric image for new generations to appreciate the fragility of our biosphere, the 3-D Digital Earths began inspiring growing numbers of people to the possibility of better understanding and possibly saving our planet. Introduction of satellite data into commercially accessible spatial toolboxes significantly advanced the capacity to map, monitor, and manage our planet’s resources and provide a unifying perspective on the Digital Earth vision.
From fall of 1998 until fall of 2000, NASA led the Digital Earth initiative in cooperation with its sister government agencies, including the Federal Geospatial Data Committee (FGDC). Attention to consensus development of standards, protocols and tools through cooperative test-bed initiatives was the primary process for advancement of this initiative within the government community. When Al Gore lost the 2000 presidential election, Digital Earth as a programmatic moniker was considered a political liability to the incoming administration and was immediately relegated to a minority status within the FGDC and used primarily to define 3-D visualization reference models. This status continues today, with a few exceptions, within the Bush administration.
With the Chinese government’s full backing and inauguration of the 1st International Symposium on Digital Earth in 1999 (see International Symposium on Digital Earth) the international community warmly supported a dialog for implementing the Digital Earth vision as articulated by Gore. In 2000, the United Nations Environment Programme (UNEP) advanced the Digital Earth to enhance decision-makers access to information for the likes of Secretary-General Kofi Annan and the United Nations Security Council. UNEP promoted use of Web-based geospatial technologies with the ability to access the world’s environmental information, in association with economic and social policy issues. The design of UNEP’s data and information resources reorganization was initiated in 2001, based on the GSDI/DE  architecture for a network of distributed and interoperable databases creating a framework of linked servers. The design concept was based upon using a growing network of internet mapping software and database content with advanced capabilities to link GIS tools and applications. UNEP.net, launched in February 2001, provided UN staff with an unparalleled facility for accessing authoritative environmental data resources and a visible example to others in the UN community. However, a universal user interface for UNEP.net, suitable for members of Security Council, that is non-scientists, did not exist. UNEP began actively testing prototypes for a UNEP geobrowser beginning in mid-2001 with a showcase for the African community displayed at the 5th African GIS Conference in Nairobi, Kenya November 2001. Keyhole Technology, Inc. (later purchased in 2004 by Google for Google Maps) was contracted to develop and demonstrate the first full globe 3-D interactive Digital Earth using web-stream data from distributed database located on servers around the planet. A concerted effort within the UN community, via the Geographic Information Working Group(UNGIWG), followed immediately, including purchase of early Keyhole systems by 2002. UNEP provided further public demonstrations for this early Digital Earth system at the World Summit on Sustainable Development in September, 2002 at Johannesburg, South Africa. In seeking an engineering approach to system-wide development of the Digital Earth model, recommendations were made at the 3rd UNGIWG Meeting, June 2002, Washington, D.C. for creating a document on the Functional User Requirements for geobrowsers. This proposal was communicated to the ISDE Secretariat in Beijing and the organizing committee for the 3rd International Symposium for Digital Earth (see International Symposium on Digital Earth) and agreement was reached by the Chinese Academy of Sciences sponsored Secretariat to host the first of the two Digital Earth geobrowser meetings.
China had fostered an explosion of Chinese Digital Earth projects and initiatives originating from its 1999 ISDE inaugural meeting. Literally, hundreds of digital earth cities had been created by the national, provincial, and municipal governments and universities in a Digital Earth space race. In China, Digital Earth became a metaphor for modernization and automation with computers leading to the incorporated of Digital Earth into the five-year modernization plan. Originating from China’s satellite remote sensing community, their Digital Earth prowess spread to a range of applications including flood predictions, dust cloud modeling, environmental assessments, and city planning. Chinese leaders at the highest levels of government have highlighted their technology as exemplified by their leveraging the Digital Olympics when successfully competing to host the 2008 Olympics in Beijing. Reliance on visualization for the Digital Olympics was accompanied by the powerful and detailed computational modeling for all aspects of the planning, including security, health, and logistics. China has been omnipresent at all international Digital Earth conferences and has recently founded the International Society for Digital Earth, one of the first NGOs created by the Chinese Academy of Sciences.
Japan, led by Keio University and JAXA, has also played a prominent international role in Digital Earth helping to create the Digital Asia Network with a secretariat located in Bangkok to promote regional cooperation and initiatives. The high-tech environment for Japanese society has been a powerful enabler for an impressive array of innovative advances using Digital Earth technology. Citizens in the Gifu Prefecture are uploading information directly into community-scale Digital Earth programs uplinked from their camera-cell phones on topics ranging from first sightings of fire-flies in spring to location of blocked handicap access ramps. Applications of the Japanese Digital Earth initiatives range from the use of the world’s largest super-computer for modeling climate change to citizen-participatory risk assessment in planning for the permanent disposal of radioactive nuclear waste.
Other nations have been aggressively proposing to host the bi-annual ISDE conferences as a reflection of the nation’s interest in Digital Earth. Recently, an Israeli author published the novel Global Dawn that provides an overview of the early initiatives to create a Digital Earth community in Israel for the middle east countries.
International Symposium on Digital Earth
The International Symposium on Digital Earth (ISDE) bi-annual conference series has been promoting advanced visualization technology under a common vision using a series of key topics or sessions. The first symposium, inaugurated by the Chinese Academy of Sciences and Chinese government, was held November 29 through 2 December 1999, in Beijing, China in 1999, with 500 participants from 27 countries (). The Beijing Declaration set in motion the bi-annual series and established the guiding principles for collaboration among the interested nations. The second symposium was held in New Brunswick, Canada June 25-27, 2001 with 700 participants and 30 countries (). The third symposium was held in Brno, Czech Republic 21-25 September 2003 (), 254 participants and 34 countries were represented. The fourth symposium held 28-31 March 2005 in Tokyo, Japan, () had 335 participants and 1,100 public attendees and 30 countries were represented. Each conference has expanded the awareness and reach of the Digital Earth vision with the exposition of cutting edge technology, such as Japan’s Super Computer. The 5th ISDE () will continue the growth of attendees and national diversity as well debuting new Digital Earth technology and network implementation. A special Digital Earth Summit on Sustainable Development was held in August 27-30 in Auckland, New Zealand due to increased awareness of Digital Earth.(). The 7th ISDE Symposium () will be held in Perth, Western Australia from 23-25 August 2011.
International Society on Digital Earth
In an unprecedented move by the Chinese government, they agreed to form a non-profit entity from the Chinese Academy of Sciences to host the international secretariat, the International Society on Digital Earth, that would help with governance and promotion of the International Symposium on Digital Earth. The first ISDE meeting was held May 21-22, 2006 in Beijing. This historic meeting marks the creation of the first sustained international governance organization for the Digital Earth vision.
Digital Earth Reference Model
The term Digital Earth Reference Model (DERM) was coined by Tim Foresman in context with a vision for an all encompassing geospatial platform as an abstract for information flow in support of Al Gore’s vision for a Digital Earth. The Digital Earth reference model seeks to facilitate and promote the use of georeferenced information from multiple sources over the Internet.  A digital Earth reference model defines a fixed global reference frame for the Earth using four principles of a digital system, namely:
- Discrete partitioning using regular or irregular cell mesh, tiling or Grid; 
- Data acquisition using signal processing theory (sampling and quantizing) for assigning binary values from continuous analog or other digital sources to the discrete cell partitions;
- An ordering or naming of cells that can provide both unique spatial indexing and geographic location address;
- A set of mathematical operations built on the indexing for algebraic, geometric, Boolean and image processing transforms, etc.
The distinction between "digital" versus "analog" Earth reference model is made in the manner the entire Earth surface is covered. Tessellation refer to a finite number of objects/cells that cover the surface as discrete partitions while Lattice refer to ordered sets of points that cover the surface in continuous vector space. The mathematical frame for a digital Earth reference model is a tessellation while the mathematical frame for an analog Earth reference is a lattice.
The value of a digital Earth reference model to encode information about the Earth is akin to the value obtained from other digital technologies, namely synchronization of the physical domain with the information domain, such as in digital audio and digital photography. Efficiencies are found in data storage, processing, integration, discovery, transmission, visualization, aggregation, and analytical, fusion and modeling transforms. Data reference to a Digital Earth Reference Model (DERM) becomes ubiquitous facilitating distributed spatial queries such as “What is here?” and “What has changed?”. Image and signal processing theory can be utilized to operate on data referenced to a DERM.
The DERM structure is data independent allowing for the general quantization of all georeferenced data sources onto the common grid. Application, algorithms and operations can then be developed on the grid independent of data sources.
Approaches using an analog reference require rigorous manual conflation to satisfy the creation of digital products such as digital maps or other cartographic, navigation or geospatial information (see also GIS). However, digital models are weaker at geometric transformations where translation, scaling and rotation must conform to the discrete cell locations wherein on an analog model with a continuum of locations geometric transformation are straight forward with no requirements for reprocessing or resampling.
A cell shape in such representations can be critical to the validity, adaptability and usefulness of the grid. As rectilinear structures are intuitive but lack optimization characteristics as a tessellation especially when tiled to a sphere, other schemes including voronoi regions, peano curves, triangles and hexagonal tilings have been advanced as superior alternatives.
Many ordering and naming models have been implemented as geospatial database indexing for efficient data retrieval (R-Trees, QTM, HHC). Few of these models have encompassed a complete digital Earth reference model where both a formation of digits that represent a hierarchy where the index contains a parent child relationship and a formation of digits that monotonically converges by a set modulus to all vector Reals.
The International Society on Digital Earth has a standing committee considering DERM implementations and standards which includes both the Earth reference frame and the ancillary requirements for metadata and attribute semantics.
Digital Earth Technologies
- US Government Digital Earth Prototypes (2001)
- ADEPT - Alexandria Digital Earth Prototype (1999-2004)
- US Government Digital Earth Reference Model (DERM)
- ISDE Digital Earth Reference Model Working Group
- Geospatial Interoperability Reference Model (GIRM)
- Global Spatial Data Model (GSDM)
- Tim Foresman converstaion with Charles Herring in New Zealand, Digital Earth Convention, 2007
- John D. Evans, NASA Digital Earth Office, June 2001 see http://www.cartome.org/draft-derm.htm
- Perry R. Peterson, Gene Girard, Charles Herring, 2006. see http://www.pyxisinnovation.com/pyxwiki/index.php?title=Digital_Earth_Reference_Model
- Sahr, K., D. White and A.J. Kimerling. 2003. "Geodesic Discrete Global Grid Systems", Cartography and Geographic Information Science, Vol 30, No. 2, pp. 121-134. see http://www.sou.edu/cs/sahr/dgg/pubs/gdggs03.pdf Survey of Discrete Global Grids