Technical basis of digital earth

Realizing digital earth on computer is not a simple matter. It needs the support of many disciplines, especially information science and technology. This mainly includes: information superhighway and computer broadband high-speed network technology, high-resolution satellite images, spatial information technology, large-capacity data processing and storage technology, scientific calculation and visualization and virtual reality technology. The data needed by the digital earth is no longer stored in a database, but needs to be maintained by thousands of different organizations. This means that servers participating in the digital earth will need to be connected through high-speed networks. Therefore, as early as February 1993, President Clinton proposed to implement the National Information Infrastructure (NII), commonly known as the information superhighway, which is mainly composed of computer servers, networks and computer terminals. The United States invested 400 billion dollars in this plan, which took 20 years. By the year 2000, the goal is to increase the productivity by 20-40% and obtain benefits of $3.5 trillion.

Driven by the explosive growth of Internet traffic, telecommunication operators have tried to use the network of 10G/S, and the Internet with10/5 bytes per second is under study. It is believed that there will be more excellent broadband high-speed networks for people to use in 2 1 century. In this century, the resolution of remote sensing satellite images has been rapidly improved in the more than 20 years since the advent of satellite remote sensing. Resolution here refers to spatial resolution, spectral resolution and temporal resolution. Spatial resolution refers to the minimum target size on the ground that can be seen on the image, which is expressed by the size of pixels on the ground. From 80 meters at the beginning of remote sensing, it has increased to 30 meters, 10 meters, 5.8 meters, or even 2 meters, and the military can even reach 10cm. It will be very convenient to acquire spatial resolution images above 1m in the next century. Spectral resolution refers to the band range of imaging. The finer the division, the more bands and the higher the spectral resolution. 2 1 century technology can reach the order of 5~6nm (nanometer), with more than 400 bands. Subdividing the spectrum can improve the ability to automatically distinguish and identify the properties and components of the target. The time resolution refers to the revisit period, and the Earth observation satellite is generally 15~25 days. By launching reasonably distributed satellite constellations, the Earth can be observed every 3-5 days.

In the next century, high-resolution satellite remote sensing images will be superior to the spatial resolution of 1 meter, providing detailed data reflecting the dynamic changes of the surface for human beings every 3-5 days, and realizing the ideal that a scholar can see the world without leaving home. Spatial information refers to information related to spatial and geographical distribution. According to statistics, 80% of things in the world are related to spatial distribution, and using spatial information for earth research is geographic information system. In order to meet the requirements of digital earth, GIS software and network GPS integrating image database, vector graphics database and digital elevation model (DEM) will be very mature and popular in the next century. Therefore, different levels of interoperability can be realized, and the geographic information generated by one GIS application software will be read by another software.

When people process, publish and query information on the digital earth, they will find that a lot of information is related to geography and spatial location. For example, inquiring about the traffic connection between the two cities, inquiring about tourist attractions and routes, and choosing a cheap and environmentally friendly house when buying a house all require geospatial reference. Because the spatial data reference framework has not been established, it is not easy to connect relevant information to geospatial reference when making home pages on the World Wide Web. Therefore, the national spatial data infrastructure is the foundation of the digital earth.

National spatial data infrastructure mainly includes spatial data coordination management and distribution systems and institutions, spatial data exchange websites, spatial data exchange standards and digital geospatial data frameworks. This was the task assigned by President Clinton in April. 1994. The United States will initially complete it in 2000 1 month. At the turn of the century, China will also pay close attention to the establishment of China spatial information infrastructure based on 1:50000 and 1: 10000 scales. Europe, Russia and the Asia-Pacific region are also grasping the spatial data infrastructure.

Spatial data sharing mechanism is one of the keys to realize digital earth. The ISO/TC2 1 1 Working Group of the International Organization for Standardization is making efforts to this end. Only sharing can develop, sharing promotes informatization, and informatization further promotes sharing. Co-construction between the government and the people is the basic condition to realize the principle of sharing, because it is impossible for any country's government to take over the whole information construction. In China, following this law, it is necessary to break the boundaries between departments and regions, unify standards, act together, coordinate with each other, understand and tolerate each other, divide labor and cooperate, and give full play to the overall advantages. Only major alliances can form economies of scale advantages and strive for initiative in the fierce international information market competition. The digital earth will need to store 10 15 bytes of information. The Planetary Earth Project EOS-AM 1 99 of NASA will generate 1000GB (1TB) of data and information every day. The resolution image of 1TB covers Guangdong province, and the data volume is about 1TB, while Guangdong is 153 in China. Therefore, if China wants to build a digital earth, the light and shadow image data will be 53TB, which is only a dynamic data of one moment and multi-phases, with a larger capacity. NASA and NOAA have set out to build a data center that can store 1800TM, which is managed by a prototype parallel machine, and the search for data tapes is completed automatically and quickly by robots. I believe there will be a new leap in the next century.

On the other hand, in order to quickly find the needed data in the massive data, it is very necessary to establish a meta-database, which is data about data, through which we can know the name, location, attributes and other information about data, thus greatly reducing the time for users to find the needed data. (1) virtual technology

Visualization is a window and tool to realize the interaction between digital earth and people. Without visualization technology, a bunch of numbers in the computer are meaningless.

A remarkable technical feature of digital earth is virtual reality technology. After the establishment of digital earth, users can see the earth emerging from space by wearing a display helmet, and use the window of user interface to open and enlarge the digital image; With the continuous improvement of resolution, he saw the mainland, then the countryside, cities, and finally private houses, shops, trees and other natural and man-made landscapes; When he is interested in goods, he can enter the store, admire the clothes in the mall, and construct a virtual fit dress according to his body shape.

Virtual reality technology provides an immersive feeling for human beings to observe nature, appreciate landscapes and understand entities. In recent years, virtual reality technology has developed rapidly. Virtual Reality Modeling Language (VRML) is a Web-oriented and object-oriented three-dimensional modeling language and an explanatory language. It not only supports the three-dimensional representation of data and processes, but also enables users to enter the virtual world with realistic audio-visual effects, thus realizing the representation of the digital earth and studying various earth phenomena and people's daily applications through the digital earth. In fact, artificial virtual reality technology is a mature technology in the field of photogrammetry. With the development of digital photogrammetry in recent years, it has become possible to establish measurable digital virtual technology on the computer. Of course, the current technology is to take pictures of the same entity, produce parallax and build a three-dimensional model, usually when the model is processed. The further development is to seamlessly splice the whole earth, roam freely and enlarge it, and construct a virtual stereo from three-dimensional data through artificial parallax.

(2)3S technology

The core of digital earth is geospatial information science, and the most basic and basic technical core of geospatial information science and technology system is 3S technology and its integration. The so-called 3S is a general term for Global Positioning System (GPS), Geographic Information System (GIS) and Remote Sensing (RS). Without the development of 3S technology, it is impossible for the changing earth to enter the computer network system digitally.

(3) Positioning technology

As a brand-new modern positioning method, GPS has gradually replaced the traditional photoelectric instruments in more and more fields. Since 1980s, especially since 1990s, the combination of GPS satellite positioning and navigation technology and modern communication technology has revolutionized space positioning technology. Using GPS to measure three-dimensional coordinates at the same time makes mapping and positioning technology extend from land and offshore to the whole ocean and outer space, from static to dynamic, from single-point positioning to local and wide-area difference, from post-processing to real-time (quasi-real-time) positioning and navigation, from absolute and relative accuracy to meters, centimeters or even submillimeters, thus greatly broadening its application scope and role in all walks of life. In the near future, everyone can wear a GPS watch, plus a mobile phone, and your activities can automatically enter the digital earth.

(4) Remote sensing technology

The development of contemporary remote sensing is mainly manifested in its characteristics of multi-sensor, high resolution and multi-temporal. 1) Multi-sensor technology. Contemporary remote sensing technology has been able to completely cover all parts of the atmospheric window. Optical remote sensing can include visible light, near infrared and short-wave infrared regions. The wavelength range of thermal infrared remote sensing can be from 8 to 14 mm, and microwave remote sensing can be used to observe the radiation and scattering of electromagnetic waves of targets, which can be divided into passive microwave remote sensing and active microwave remote sensing, and the wavelength range is from 1 mm to 100 cm.

2) High resolution characteristics of remote sensing. Fully reflected in three aspects: spatial resolution, spectral resolution and temperature resolution, the spatial resolution of long-line array CCD imaging scanner can reach 1~2m, and the spectral subdivision of imaging spectrometer can reach the level of 5~6nm. The temperature resolution of thermal infrared radiometer can be improved from 0.5K to 0.3K or even 0.1k.

3) Multi-temporal characteristics of remote sensing. With the implementation of the small satellite group plan, multiple small satellites can be used to repeatedly sample the surface every 2-3 days to obtain high-resolution imaging spectrometer data. Multi-band and multi-polarization radar satellites will be able to solve all-weather and all-day earth observation under rain and fog conditions. The organic combination of satellite remote sensing and airborne and vehicular remote sensing technology is a powerful guarantee for the acquisition of multi-temporal remote sensing data.

The application analysis of remote sensing information has changed from single remote sensing data to multi-temporal and multi-data source fusion analysis, from static analysis to dynamic monitoring, from qualitative investigation of resources and environment to computer-aided quantitative automatic mapping, from surface description of various phenomena to software analysis and quantitative exploration. Aerial remote sensing has become an important aspect of remote sensing development because of its remarkable characteristics of fast maneuverability and high resolution.

(5)GIS technology

With the introduction of the concept of digital earth and the deepening of people's understanding of it, the development from two-dimensional to multi-dimensional dynamics and networking is the main direction of the development of geographic information system, and it is also an urgent need for the theoretical development of geographic information system and many fields such as resources, environment and cities. In terms of technology development, one development is based on the client/server structure, that is, users can call the data and programs on the server on their own terminals. Another development is to develop Internet GIS or Web-GIS through the Internet, which can realize remote search of various geospatial data, including graphics and images, and can carry out various geospatial analysis. This development is to further combine geographic information system with information superhighway through modern communication technology. Another development direction is data mining, which automatically discovers knowledge from spatial database to support the automation of remote sensing interpretation and the intelligence of GIS spatial analysis.

(6) integration technology

3s integration refers to the organic integration of the above three new Earth observation technologies and other related technologies. The Integration mentioned here is the Chinese translation of English integration, which refers to an organic combination, online connection, real-time processing and complete system. The integration of GPS, RS and GIS can be realized at different technical levels. 3s integration includes space-based 3s integration and ground-based 3s integration.

Space-based 3s integration: direct observation of the earth is realized by using the positioning mode of heaven and earth. The main purpose is to realize direct positioning, reconnaissance, guidance and measurement of space remote sensing information without ground control points (or with a small number of ground control points).

Ground-based 3s integration: vehicle-borne and ship-borne positioning and navigation, as well as real-time operations such as positioning, tracking and measuring ground targets.