The so-called WebGIS, in popular terms, is the combination of geographic information systems and the Internet. Through Internet technology, spatial data can be shared on the Web, allowing users to browse and use these spatial data. In WebGIS, through a node of WW1N, Internet users can access various spatial data in WebGIS sites, and can also perform thematic map creation, spatial search and spatial analysis. In summary, WebGIS is a network information system formed by Intenet/Intranet, which consists of a host, a database, and a terminal. In WebGIS, the GIS function layer and the data management layer are the network client side, and various information is acquired and applied through the client side. In addition, the network server side is the information data maintenance layer, mainly providing information data and performing system services. Therefore, the WebGIS system is mainly divided into four parts: a WebGIS browser for displaying spatial data information and performing online processing on the client side; a WebG1S information agent for balancing network load and facilitating more spatial information of the network; There is a We-bGIS server for processing client query requests and spatial analysis requests; finally, the WebGIS editor is mainly for spatial data processing.
The development of the WebGIS project is mainly the B/S architecture. The most popular is client-side javascript, server-side java. There is also a flex client.
The client mainly completes user interaction, sends requests to the server and transmits parameters, and organizes the results returned by the server. The server side completes the database additions and deletions, business functions, and so on.
The bottom layer of the server is the data layer, which completes the addition, deletion and modification of data. Commonly used databases are oracle, mysql, etc. The most basic method is that the java language uses jdbc to organize the corresponding SQL statements to implement database operations. A more convenient way is to use a third-party framework such as hibernate. Hibernate implements the mapping of database tables and Java class objects orm, using the hql statement to facilitate database operations.
Above the data layer is the logical transaction layer. The logical transaction layer corresponds to each functional module, and the functional module implements a specific function. The bottom layer of the functional module generally includes a call to the database DAO class. This layer contains special classes that accept client requests, such as servlet classes, acTIon classes, and more. Available frameworks are spring, structs2, and so on.
The client is made up of interfaces and scripts (response functions). The combination of html+javascript: The interface is rendered by a combination of div and other tags +css style; the script completes the response of the interface initialization, click, jump and other events.
One of the most important tasks of the client is to send a request to the server and get the response. I have to mention AJAX here. Different js frameworks provide their own AJAX implementations. Dojo provides xhrPost, xhrGet and other implementations to submit requests, pass parameters, get response results and handle them accordingly.
The client can also get the desired result by calling webservice.
WebG1S implementation technology analysisWith the development of WebGIS, the commonly used WebGIS implementation technologies are: public gateway interface, which is CGI; server application interface, commonly referred to as ServerAPI; dynamic server page (AcTIveServerPage) and plugin method (Plugins), client control (AeTIveXContro1) and Java technology. The following is a separate introduction and analysis.
CGl technologyThe Common Gateway Interface (CommonGateInterface) is a CGI, which is a standard interface for WWW server hosts to perform various external services. CGI technology is one of the earliest technical methods for web dynamics and interactivity enhancement. When web interaction occurs, the remote client uses the browser to perform data operation requests, and then the server understands the data request through the background CGI program. The data is processed and the processing results are finally fed back to the remote user. The use of CGI technology for WebServer expansion, the operation is much simpler, the call to the program can only be carried out through a simple command parameter, and secondly, the CGI program can also run independently, making it easier to debug. At the same time, the network server is strict in terms of CGI access control, and it also ensures the security of the system. Finally, applications that are standardized by CGI technology can be ported on WebServer, which provides great convenience for data manipulation of computer platforms. However, the WebGIS of the CGI technology model also has certain flaws.
(1) Performance aspects.
When applying CGI technology, in the face of a heavy load request environment, CGI often has poor performance. In the CGI technology environment, in the face of a new request, Server will build a new process for the request. The load caused by the construction of these new programs puts a heavy burden on the operation of the Server.
(2) Operating status.
In WebGIS applications, different CGI programs cannot distinguish between data sharing. Therefore, each program process is relatively independent, and cannot use internal storage variables to transfer information. The use of external storage technology for information transfer has seriously affected the running status of webCCI.
(3) The network transmission load is large.
In CGI technology applications, the spatial data information transmitted by the server and the client is images in JPEG and CIF format, which directly affects the visual operation and analysis of spatial data. When the client performs spatial data transmission, the necessary connection cannot be formed between multiple data transmissions, which directly causes the network transmission load to increase.
ServerAPI technologyCompared to CGI technology, ServerAPI is more effective for WebServer extensions. In the ServerAPI environment, the original CGI program was replaced by an application function, such as Microsoft's ISAPI. The application function belongs to a part of the shared library. The shared object is only loaded once. After the loading is completed, the loaded content occupies an address space in the server. When the add-on requests again, only one function call is needed. Therefore, compared to CGI technology, ServerAPI's expansion of the process greatly reduces the communication load caused by program creation, and therefore, it has a faster running speed. However, in the ServerAPI technology environment, the client cannot perform data management well. Therefore, in terms of WebCGI technology solution, the ServerAPI technology also has the drawback of large network transmission load.
ASP technologyAcTIveServerPage technology is a server-side scripting environment developed by Microsoft. It can use ASP technology to combine HTML web pages, ASP commands and ActiveX controls to create a dynamic, interactive and efficient application. In the web server application, the server side is used for ASP file storage, and when the client sends a request to the ASP file, the server side interprets the ASP file, and then forms an HTML web page, which is fed back and delivered to the client. Compared with CGI technology, ASP technology has effectively improved the interface objectization, and automatically analyzed various collected data, which has achieved good data processing results. Finally, ASP technology has the same defects as CCI and ServerAPI technologies, and does not effectively solve the problem of network transmission load. When conducting WebGIS network transmission, it also bears a lot of pressure.
Plug-in and ActiveXControl technologyPlu in and ActiveXControl are commonly referred to as plug-ins and controls. Plug-in and ActiveXControl technologies can be used to extend the functionality of the browser to analyze and interpret some custom GIS data file formats. For example, the Plug-in technology can automatically load the Plu in dynamic link library during the browser startup process. When a certain format element is embedded in the browser access page, the browser can find a matching plugin in the plugin link library. And then analyze and explain the format elements. Therefore, the plug-in technology uses local code to perform data execution, and it runs fast. In terms of browser processing functions, plug-ins and space technologies can also perform vector map data processing. At the same time, plug-in and space technology begin to have client-side logic to a certain extent, so that load balancing between the client and the server can be achieved, reducing network bandwidth pressure. However, for the user, it is necessary to install the browser embedded function module on the local machine, which causes great inconvenience and is not safe enough.
Java languageJava technology was born in 1994, and the Java language has been widely promoted since its launch. The Java language is simple to use, operability, dynamic, and secure. It can also be ported. It is a cross-platform programming language suitable for distributed computers. The Java language has high compatibility with various resources on the network platform. Therefore, the Java language is very suitable for the Internet and distributed language environments. By downloading a Java program on the client side, various operations can be run on the server. Therefore, the Java language is by far the most ideal technology for developing and applying webGIS technology. There are two main types of development and implementation of the Java language in WebGIS.
(1) A WebGIS system that only uses Java technology on the client side.
When only the client adopts Java technology, the server side implements the Java program interaction function through the CGI data transfer protocol based on the existing system code. In this case, system development is quite simple, the development time of the system and program is greatly shortened, and the function of the system can be guaranteed to meet the needs of mapping and geospatial analysis, and at the same time, to a certain extent. Cross-platform services.
(2) We-bGIS system using Java technology for both client and server
In this mode, the advantages of Java language technology can be maximized, and the advantage of providing technical support for establishing a service network between the client and the server by means of Java technology can well realize the development of the WebGIS system. However, it is worth noting that a large number of systems are currently based on c/c++ GIS systems. Therefore, it is necessary to develop corresponding Java versions for these systems, which directly causes the difficulty of system development. At the same time, when the Java technology system is developed, the slow execution speed of the program is also a problem. In order to solve this problem, some software companies solve the problem by the compiler, translate the bytecode before the execution of the Java program, and then use the CPU. Running, greatly improving the program execution speed of Java technology. Therefore, compared with the traditional c/c++ program, Java is greatly shortened in development time, application performance is greatly improved, and system resources occupied during file execution are relatively small. At the same time, Java technology has a good cross-platform advantage, and it also has good advantages in terms of technical support and execution speed. Using Java technology for WebGIS development can be compatible with various resources, and it is very important to promote the operation of the browser and perform various spatial data operations. It is very important for the development of WebGIS technology.
Through the introduction of various WebGIS development technologies, the advantages and disadvantages of CGI, ServerAPI, ASP and other technologies are analyzed. Finally, it is a good choice to develop and apply WebGIS system using Java technology. It is a WebGIS system development through Java technology. And development provides effective technical support, facilitates the use of the system for users, promotes the development of geographic information systems, and can also realize the development of WebGIS in the direction of popularization and socialization.
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