In order to solve the problems of digitalization, informatization and application in digital earth, the following key technologies need to be studied: scientific computing, mass storage, broadband network, satellite data acquisition, metadata, interoperability and so on [Gore]. The following is a brief description of these key technologies.

1. Fast acquisition technology of high resolution satellite remote sensing data: Satellite remote sensing is the main means of data acquisition for digital earth, including land satellite series, ocean satellite series, meteorological satellite series and small satellite series with different altitudes and resolutions. Its resolution ranges from 1 meter to 4 000 meter. Remote sensing data processing includes radiation correction, geometric correction, enhancement, feature extraction, automatic classification, automatic mapping, data compression and so on. High-resolution satellites produce a large amount of data every day. Automatic and fast processing of these data to extract information in real time and accurately is the key to achieve digital earth information acquisition.

2. Storage and processing of geospatial data: In order to digitize the information on the earth, besides storing and processing a large number of remote sensing data, it also includes graphic data, attribute data and so on. To realize the query and retrieval of these data, massive data storage management and fast processing technology are needed. At present, distributed data storage is the trend of massive data management, which can avoid the management difficulties and network congestion caused by centralized systems. Usually, supercomputers or parallel computing are used to achieve fast processing.

3. Hypermedia spatial information system: One of the main tasks of Digital Earth is to share and publish information through the Internet, mainly through WebGIS technology. In addition, digital earth applications contain a large number of multimedia data, which also need to be published on the Internet to form a hypermedia spatial information system. A large number of data are transmitted on the network, causing network congestion, which requires a high bandwidth network to solve this problem.

4. Distributed computing of geographic information: Geographic information is distributed and basic, shared and comprehensive. Distributed computing can make geographic information applied in all areas of society. Following the OpenGIS specification and based on CORBA (or COM) architecture, it is one of the solutions to realize the distributed calculation of geographic information. Distributed computing services for geographic information include:

   1. Geographic information sharing services: providing geographic information services for applications in various fields of society, providing a foundation for sharing and integration.

   2. Spatial query service: including information retrieval through metadata, querying various aspects such as attributes, geometry, spatial relationships, etc.

   3. Spatial analysis services: Providing services such as spatial information extraction, geographic feature analysis, image understanding, and image development.

   4. Spatial mapping services: including spatial coordinate conversion services, geographic annotation services, image processing services, feature synthesis services, image map creation services, etc., with the aim of visually representing spatial information.

   5. Specific task services for geographic information: Providing targeted services for specific users in the field of geographic information, mainly for the acquisition, establishment, and maintenance of geographic information, known as geographic data management services. Including geographic information production, geographic information production management, map symbol management, information development, utilization, and analysis, ordering and tracking, information storage and retrieval systems, information delivery, information reproduction and replication, and user support.

5. Incomplete scale database:The incomparable-scale database refers to a large-scale database as the basic data source. The information of spatial objects in a certain area increases and decreases automatically with the change of scale. That is to say, it can automatically generate smaller-scale data from large-scale spatial data. Automated cartographic generalization is the key technology of incomparable scale database. There are many studies in this field, but it is difficult to achieve satisfactory results.

6. Spatial data warehouse: Spatial data warehouse is a collection of thematic, integrated, time-varying, persistent and spatial coordinates supporting management and decision-making processes. The main task of data warehouse is to standardize, filter and match, refine, label timestamps and confirm data quality for raw data with different sources, structures and formats. Then, according to the needs of tasks, data integration and segmentation, generalization and aggregation, prediction and derivation, translation and formatting, transformation and re-mapping are carried out. Finally, the data warehouse is modeled, summarized, aggregated, adjusted and validated, and structured queries are established. The purpose of spatial data warehouse is to process the accumulated massive spatial data, extract useful information and provide decision support. Its framework includes: data source, metadata data source, metadata interoperability protocol, data extraction and refinement, metadata creation and browsing data warehouse, access and retrieval, metadata management and query and analysis, etc.

7. Spatial data fusion: Spatial data fusion refers to a new kind of synthetic data, such as pseudo-color synthetic image. Many kinds of data are synthesized and the original data are no longer saved. The data fusion of digital earth includes multi-resolution data, multi-dimensional data and different types of data. The fusion data need to be visualized. Usually, the data are superimposed on the digital elevation model to form three-dimensional landscape image. To realize spatial data fusion in digital earth, geographic data interoperability and high-speed network support are needed.

8. Virtual reality technology: It refers to the use of computer technology to generate a realistic, interactive and dynamic “world” with visual, auditory and tactile effects. People can manipulate and inspect the virtual entities in the virtual world. The interaction between users and virtual reality system is carried out by using data gloves, data helmet, data clothes and so on. VR system uses visual descriptor. Audio descriptor and tactile descriptor make users feel immersive. Using virtual reality technology in digital earth, the real geographic area can be expressed very real, and users can move freely in and out of the selected geographic belt. At present, VRML (Virtual Reality Modeling Language) is one of the ways to combine GIS, virtual reality and Web technology. By describing GIS information with VRML, spatial three-dimensional data can be published on the Internet for users to browse.

9. Metadata: In the process of creating digital earth, the global demand for digital geographic information is increasing. Many units and individuals begin to produce, process and modify geographic data. In addition, in computer information system, in order to ensure that the information is not misused, detailed description of data is needed through Metadata, which is not only useful for the research of geographic entities, but also for the research of geographic entities using models. Data producers can fully describe data sets, and users can also estimate the applicability of data sets to their application purposes. Therefore, with the increase of producers and users of geospatial data, using Metadata to describe data will become an inevitable trend.

To sum up, the key of digital earth technology is to achieve massive data acquisition, storage management, processing and information extraction, information sharing and information expression. The relationship between them and the above specific technologies are given below (Table 19-1).

Table 19-1: Summary of digital earth technology

Description: indicates the specific technical support to achieve the corresponding purpose.