1. 1 Acquisition and processing of spatial information
Techniques for obtaining spatial information include: field total station survey, GPS survey, map scanning digitization, digital photogrammetry, remote sensing image target measurement, etc. Field total station survey and GPS survey software have been basically popularized.
The digital technology of map scanning and the technology of transforming it into vector database are becoming more and more mature and commercialized, such as ArcScan of ESRI Company. At present, most technologies combine interaction and automation, which can automatically track the boundaries between single lines and polygons, automatically identify breakpoints, dotted lines and symbolic lines, automatically straighten angles, and edit grid-vector integration during interaction. Although scanning digitization greatly improves the efficiency and accuracy of graphic data input, editing and attribute data input after digitization are still very heavy.
The integration of GPS into GIS and the application of GIS in the field have made great progress in the real-time collection of field data. Remote sensing image is being used as the basic map, making it the most important layer of GIS.
The technology of automatically obtaining DEM and digital orthophoto through digital photogrammetry and manually obtaining vector line drawing data has been widely used. At home, this technology is at the world's leading level, with instruments, equipment and software exported, and it also undertakes the task of foreign data acquisition.
The technology of making digital orthophoto images by remote sensing and extracting targets by interactive methods has also been basically mature, resulting in a large number of remote sensing digital orthophoto images.
In the aspect of spatial information acquisition, the remaining problems are automatic recognition and automatic measurement of ground objects, including element recognition of scanned maps, digital photogrammetry and automatic extraction of remote sensing targets. This is a subject that needs long-term research, and it is difficult to make a breakthrough in the short term.
Technically, the methods and technologies of spatial data processing are basically mature, but there is still a lack of special software for spatial data processing with high efficiency and good automation.
Another development trend of spatial data acquisition and processing is networked spatial data production. It refers to the collection and processing of spatial data based on local area network environment, using network data production management software for production scheduling, monitoring and quality control, in order to improve the production efficiency of spatial data and ensure the safety of data.
With the development of new sensors, the acquisition equipment and technology of spatial data information sources are in a period of rapid development. A number of new aerial survey and remote sensing equipment, such as laser scanning radar, high-resolution digital photogrammetry camera, infrared camera and interferometric radar, will enrich the spatial information we obtain.
1.2 spatial data storage and retrieval
GIS spatial data management has gone out of the mode of file management. The original GIS software generally used file method to manage vector graphic data and relational database management system to manage attribute data. At present, the mainstream GIS software adopts commercial relational database management system to manage graphics and attribute data. Such as ARC/INFO and GEOMEDIA abroad, and GEOSTAR, MAPGIS and SUPERMAP at home.
In data query and access, standard SQL commands are used to access and manipulate data (including adding, deleting and modifying data). In order to improve the query speed, most spatial indexing technologies such as quadtree and R-tree are introduced.
1.3 data processing and analysis
The problem of GIS in this respect is that mathematical experts who are proficient in analysis and modeling technology know little about GIS, while GIS developers often know little about spatial data analysis, modeling and spatial statistics. In the standard commercial system, there is still no basic universal spatial analysis program or basic universal modeling tool. It is worth noting that GIS vendors are incorporating raster data processing functions into their products and providing them to users as a separate module, such as Vertical Mapper of MapInfo Company.
1.4 data output
The most exciting progress of GIS in data output is that with the application of Internet and WWW technology, GIS geographic information and map data output span time and space. Any user can access GIS installed on the Web server anytime and anywhere through the Internet, and can obtain map information, make thematic maps, conduct geographic analysis and so on. On his own customized interface. It should be said that the commercial WebGIS is still in its infancy, and the query and analysis functions provided by WebGIS can not meet the needs of professional applications. However, the appearance of WebGIS began to change the traditional data output and map publishing methods of GIS, making it possible for geographic information to be highly socialized.
2. Development trend of1WebGIS
WebGIS is a network interoperability application system based on the existing Internet/Intranet architecture. It can use the Internet to publish spatial data on the Web and provide users with the functions of browsing, querying and analyzing spatial data. On the one hand, WebGIS can provide the public with online information services related to spatial information, such as transportation, tourism, catering, entertainment, real estate and shopping. On the other hand, WebGIS can provide services for enterprise internal business management based on Intranet, such as helping enterprises with equipment management, line management and safety monitoring management. With the wide application of WebGIS, it has become an inevitable trend of international GIS development. Through WebGIS, people can conveniently browse or obtain all kinds of distributed geospatial data on the Web from any node on the WWW, and conduct all kinds of online geospatial analysis.
2.2 the characteristics of WebGIS
1) wider access.
2) Platform independence. No matter what kind of machine the server/client is, no matter what kind of GIS software the WebGIS server uses, users can use a general Web browser to transparently access WebGIS data, and carry out dynamic combination of distributed components and collaborative processing and analysis of spatial data on this machine or a server, thus realizing * * * sharing of remote heterogeneous databases.
3) The system cost can be greatly reduced.
4) The operation is simpler.
5) Balance the efficient computing load. It can make full use of network resources and entrust the basic and overall processing to the server, while the simple operation with small data volume is directly completed by the client.
2.3 WebGIS implementation mode
1) server-side policy. WebGIS based on server usually adopts CGI technology, and relies on the server to complete GIS analysis and output. Every GIS operation of the client must be accepted by the server, and the corresponding CGI program must be started to process it, and then the result will be returned to the user in the form of JPEG or GIF bitmap.
2) Customer policy. Send the client program running on the local computer to the client through the server. This program can interact with users and handle some simple requests from users, such as opening windows, enlarging maps and applying for vector data directly from the server. When the client sends out some complex advanced operation requirements that the client program can't handle, it requests the WebGIS server to process them, and the processing results are sent to the client in the form of vector data.
3) Mixed strategy. Considering the computing power and network traffic of client and server comprehensively, the GIS tasks are reasonably allocated, and the computing functions of client and server are fully utilized to improve interoperability and system performance. For example, spatial database query, spatial data management and complex spatial analysis functions should be arranged on the server; User's interactive operation and control, local spatial query and theme analysis of web pages are all carried out on the client. In this way, both the client and the server have completed the task of GIS and improved the system performance.
2.4 WebGIS implementation technology
1)CGI (common gateway interface method). CGI technology is the earliest method used by WebGIS. CGI is a standard technology to connect application software and web server, and it is a functional extension of HTML.
2)ServerAPI (server application program interface). ServerAPI is a more effective extension method of WebServer than CGI, which greatly reduces the load of process creation and inter-process communication, and runs much faster than CGI programs.
3)ASP (Dynamic Server Page). ASP solves the objectification problem of CGI interface and can automatically analyze the data collected by web pages. At the same time, ASP can use other ActiveX objects in Windows environment.
4) Plug-ins and ActiveX controls. Plug-ins and ActiveXControl are methods to expand the browser's functions and enable it to interpret the custom GIS data file format.
The advantages of this method are: fast execution; Can process vector map data; To a certain extent, the load on both ends of the client and server is balanced, and the requirement for network bandwidth is reduced. However, the embedded function module of this browser needs to be installed on the local machine, which is inconvenient and unsafe for customers. At the same time, the compatibility and version management problems between different versions in traditional software programming methods can not be solved. Once the new format is established, the embedded module in the corresponding browser must be reinstalled.
5)Java .Java has become the most ideal development language to realize the distributed application architecture of WebGIS. At present, there are two ways to develop WebGIS system with Ja va: one is a WebGIS system in which only the client part adopts Java technology, and the server part uses functional modules to formulate GIS spatial data transmission protocol and interact with Java programs on the basis of the existing system code, which is the method adopted by most WebGIS systems at present. Its characteristic is that the system development is simple and easy, which can greatly shorten the system development cycle. At the same time, it can ensure that the developed system has strong mapping and geospatial analysis capabilities, and can achieve cross-platform application to a certain extent. The second method is that both the client and the server are based on We bGIS of Java. That is WebGIS of pure Java system. This development method can give full play to the advantages of Java technology, especially the supporting technology provided by Java for building distributed network applications on server and client.
3. 1 development trend of geographic information system
GIS (C/S) has experienced the evolution from project GIS, department GIS, enterprise GIS and social GIS, and its system integration has also experienced the changes from host GIS, (traditional GIS), distributed GIS(C/S), intelligent GIS(WebGIS) and virtual GIS. It can be seen that GIS is always developing in the direction of higher performance, lower cost, more openness and more flexibility. With the maturity of object-oriented theory and method, the gradual improvement of virtual reality technology and the popularization of network and intelligent system, the integration strategy of WebGIS system based on Internet and Intranet will be the mainstream technology of 2 1 century GIS system.
3.2 XML-based open spatial data exchange format in network environment
XML (Extensible Markup Language) allows information providers to define tags and attribute names by themselves as needed, and can also contain descriptions, so that the structure of XML files can be complicated to any degree. XML is cross-platform, open, extensible and highly structured.
Geography markup language GML(Geography Markup Language) is a coding specification based on XML, which is formulated by OpenGIS Alliance and used for the transmission and storage of geographic information (including the geometry and attributes of geographic elements). It describes the world with geographical features and can encode very complex geographical entities.
3.3 Open Geographic Information System
The essential feature of the network is its openness. Therefore, the architecture of WebGIS should be open, interoperable, upgradeable and extensible. Open WebGIS should first include the opening of data, that is, the sharing of information distributed in heterogeneous databases. The emergence of XML provides a good solution. In addition, it should also include the openness of data access, that is, there is good interoperability between different GIS software. These requirements for WebGIS are the purpose of OpenGIS alliance.
Compared with traditional GIS, OpenGIS has established a general technical foundation for open geographic information processing. It has the characteristics of interoperability, expansibility, openness, portability, compatibility, realizability and synergy.
3.4 WebGIS based on distributed computing
At present, distributed computing only realizes client/server computing, which is an intermediate step to realize fully distributed computing. Fully distributed computing is a decentralized and peer-to-peer collaborative computing, and it is an ideal computing model in the next century.
At present, the architectures or standards adopted by distributed computing platforms include CORBA and the object request agent architecture of the same object management group; Microsoft's distributed component object model DCOM and distributed network architecture DNA；; Distributed computing environment and SUN's Java.
The application of distributed WebGIS has developed from simply displaying the drawn map on the Web browser to the integration of GIS functions based on the Internet. Remote GIS users can enjoy ordinary GIS data and communicate with other GIS users in real time. The development of distributed InternetGIS application technology mainly focuses on three aspects: server, client and network communication.
3.5 Network Virtual Geographic Environment
Three-dimensional virtual reality technology is becoming a hot spot in network application. With the rapid development of Internet and the maturity of three-dimensional technology, people no longer meet the interactive characteristics of two-dimensional space on web pages, but hope to turn WWW into a three-dimensional space.
The visualization provided by virtual geographic environment (VR) technology is not only the spatial display of general geometric shapes, but also the visualization of geographic information, noise, temperature change, force change, wear and vibration. But also can show people's innovative thinking as a visual virtual entity, and promote people's creative inspiration to further sublimate.
Geographic Virtual Modeling Language (GeoVRML) describes geospatial data based on Virtual Modeling Language (VRML). Its purpose is to let users browse geo-referenced data, maps and 3D terrain models through standard VRML plug-ins installed on Web browsers. Its appearance will provide a good data specification platform for realizing the virtual geographical environment under the network environment, and will greatly promote the application of the network virtual geographical environment.
3.6 mobile GIS
Mobile GIS is an application service system. In a narrow sense, mobile GIS refers to GIS that runs on mobile terminals (such as PDA) and has the function of desktop GIS. It has no interaction with the server and is in offline operation mode. Generalized mobile geographic information system is an integrated system, which integrates geographic information system, global positioning system, mobile communication, Internet service and multimedia technology. Mobile GIS has the following characteristics:
1) Mobile GIS runs on various mobile terminals. It can interact with the server through wireless communication and obtain spatial data in real time. It can also run independently without the limitation of the server and transmission media, and has mobility.
2) As an application service system, mobile GIS should be able to respond to users' requests in time and deal with the real-time influence of time-varying factors in users' environment, and it is dynamic (real-time).
3) Mobile GIS integrates various positioning technologies to determine the user's current location and related information in real time, so it is dependent on location information.
4) Mobile GIS is displayed on mobile terminals, such as mobile phones, PDA, vehicle terminals, etc. The manufacturers of these devices are not unique, and the technologies adopted are not uniform, which will inevitably lead to the diversity of mobile terminals.
3.7 3D Geographic Information System
Traditional GIS is two-dimensional, and can only process and manage two-dimensional graphics and attribute data. Some softwares also have the function of 2.5-dimensional DEM terrain analysis. With the development of science and technology, 3D modeling and 3D GIS have developed rapidly, which has great market attraction.
Real 3D GIS should not only express three-dimensional objects (the surface of the ground and buildings on the ground), but also express the interior of objects, such as mines and groundwater. Because geological ore bodies, mines and other three-dimensional entities not only have irregular surfaces, but also have different internal materials, the z value can only be used as spatial coordinates instead of attributes. The value of any point in the ore body is a function of three-dimensional coordinates x, y, z, that is, P=f(x, y, z). At present, when we are doing three-dimensional visualization, z is a function of xy. How to visualize P=f(x, y, z) to show the surface morphology and reflect the internal structure of ore bodies is a difficult problem. Therefore, the current true 3D GIS is still a "bottleneck" problem, and some practical systems have been introduced, but they are generally simplified.
Conclusion:
Generally speaking, GIS presents the development trend of networking, openness, virtual reality, integration and spatial multi-dimension. As a computer-based application tool, GIS combines the functions of map visualization and spatial geographic analysis with the functions of database, providing an intelligent means to analyze, synthesize and query spatial data, which involves the mutual penetration and support of multiple disciplines.