WebGIS - World Wide Web Geographic Information System

Internet and GIS

Undoubtedly, the development of the Internet has brought great convenience to the development of GIS, at the same time, it also provides a new field for the research of GIS theory and technology. As an information system and a discipline, GIS can be affected by the development of the Internet in the following aspects:

  1. GIS researchers use newsgroups or e-mail to discuss the technical problems of GIS.

  2. Network distance education, that is, professors put teaching plans on the Internet in the form of HTML documents, students download and use them, and can use e-mail to ask questions, thus forming a “Virtual University”. The education of GIS can also be carried out in this way, at present, there are some websites about GIS education, such as http://gis www.kingston.ac.uk. [1]_ Three topics related to GIS are provided; the website http://www.utexas.edu introduces the challenges faced by GIS and geography teaching, the website http://www.census.gov/pub/geo/gis-faq.txt has many questions and answers on common GIS questions.

  3. The download of GIS software, the latest version of its development software can be placed on its website regularly for users to download and try out.

Some of the URLs listed in this article are very likely to be “invalid” when the reader sees it, which is almost common on the Web.

WWW website introduction of typical geographic information system[Gong Peng]

(1)GIS WORLD INC. *http://www.gisworld.com*

GIS World is currently the best-selling magazine on GIS in the world, its website lists the content catalogue of any issue published by the company, users can also search for content by using the search engine.

(2) GIS software developer website

ESRI Corporation (http://www.esri.com), Intergraph Corporation (http://www.intergraph.com), MapInfo Corporation (http://www.mapinfo.com) .

These are considered top-notch GIS sites and have a wealth of GIS information, including: company presentations, products, company publications, annual reports, conference forecasts, user information, product orders, technical consulting, programming skills, employment opportunities. , product display and free software downloads, etc.

(3) Virtual Traveler Network *http://www.vtouris.com*

The site allows users to interact with the map display, first the world map, then the map of countries or regions, and finally the city map, the map is a raster image, realize user interaction with spatial location, there are a large number of text, image and other descriptive information for users to consult.

(4) Canada Earth Information Science Network *http://abbott.ccm.emr.ca*

The website is a national-level management organization for earth information, which provides a large number of products and services, including satellite images of Canada, National Atlas data, Canadian topographic database, topographic database, road network, digital topographic data and so on. In addition, it provides the service of obtaining the above data, and uses the searcher to get the data of the search area.

(5) Filled data query and expression URL*http://tiger.census.gov/cgi-bin/mapbrowse-tbl*

The site uses fill-in-the-blanks for queries. It provides a form, after the user fills in the relevant information, the request is sent to the server-side GIS, cartography, database query and other software to run the corresponding search and query, or it can be spatial analysis, etc., the result can be a map or a table, and the result Go back to the browser. Microsoft’s portal site MSN also provides a similar feature http://msn.maps.expedia.com/quickmaps.asp.

(6)GIS Forum *http://gis.itc.nrcs.usda.gov/gishome.html*

The website includes the publication of GIS data and software, the events in the field of GIS, the discussion of some technical issues, the introduction of GIS books, etc., in addition, related issues can be discussed by means of Email.

4)Spatial data publishing and downloading, data is the most important part of GIS system, data input and pre-processing is also the most time-consuming and capital-consuming link in the process of GIS application development, and data sharing through the Internet can reduce the development cost of GIS engineering. Due to the rapid development of the Internet, the rise of e-commerce has been promoted, and spatial data can of course be sold on the Internet as a special commodity. Compared with the WebGIS method mentioned later, the data download here is mainly realized by means of file transfer. Due to the sheer volume of information on the Internet, which often becomes difficult to find the data that is really needed, and application space metadata allows users to quickly locate the required data and download it. In 1994, the U.S. government began to develop the National Spatial Data Infrastructure (NSDI), by defining metadata standards, government agencies at all levels are required to use metadata to describe the data they produce on the Internet, so as to achieve the goal of data production and sharing among agencies.

5)In addition, due to the development of the Internet, it has broken the traditional way of time-space connection and formed a new organizational form of space things, called Cyber Space, which is a hot topic in the study of human geography and will also be an important topic in the study of GIS.

WebGIS introduction

The most exciting product of the combination of Web technology and GIS technology is WebGIS (World Wide Web Geographic Information System). WebGIS, in short, is a new technology of expanding and perfecting geographic information system by using Web technology. Because HTTP protocol adopts request/response mechanism based on C/S,which has strong user interaction ability, and can transmit and display multimedia data in browser, the information in GIS mainly needs spatial data expressed in graphics and images, users query and analyze spatial data through interactive operation. These characteristics make it possible for people to use the Web to find the spatial data they need and perform various operations. Specifically, the application of WebGIS can be divided into the following aspects:

1)Spatial data release

Because it can display spatial data graphically, WebGIS makes it easier for users to find needed data than FTP.

2)Spatial query retrieval

Using the interactive ability provided by the browser, the query and retrieval of the graphics and attribute database are carried out.

3)Space model service

In the server side, various spatial models are provided, after receiving the model parameters input by the user through the browser, the calculation results are returned. In other words, using the Web can not only publish spatial data, but also publish spatial model services to form browser/server structure (B/S).

4)Organization of web resources

On the Web, there are a lot of information, most of which have spatial distribution characteristics, for example, distributor data often have location attributes, maps are used to organize and manage these information, and provide users with spatial-based retrieval services, which can undoubtedly be realized by WebGIS.

Compared with the traditional geographic information system, WebGIS has its own special features, which are mainly manifested in:

  1. It must be a network-based client/server system, while the traditional GIS is mostly a stand-alone system.

  2. It uses the Internet to exchange information between clients and servers, which means that the transmission of information is global.

  3. It is a distributed system, users and servers can be distributed in different places and different computer platforms.

Implementation technology of WebGIS

WebGIS is an important part of WebGIS. Some concepts of WebGIS, such as client/server mode and distributed data management, which can also be applied to WebGIS. However, in the implementation of WebGIS, two issues should be considered, namely, controlling the amount of data transmitted through the network and interacting with users through browsers.

At present, there are many different technologies and methods used in the development and implementation of WebGIS, including CGI (Common Gateway Interface), Server API, Plug-ins, Java Applet and ActiveX. The following technologies are briefly described and compared. [Xia Fuxiang]

CGI method

CGI is a specific standard used between Web server and client browser. It allows web users to start a program (called CGI program) that exists in the host of web server through the command of web page and receive the output of the program. CGI is the first technology to realize dynamic web pages, which enables users to interact with each other through browsers and get the corresponding results.

CGI can be used to generate images and then transfer them to the client browser (most visitor counters on home pages are implemented by CGI programs at present). In this way, theoretically, any GIS software can be connected to the Web through CGI. Remote users send requests through browsers, and servers pass requests to the back-end GIS software, the GIS software generates a digital image according to the requirements and returns it to remote users.

In fact, due to design reasons, most GIS software can not be directly connected to the Web as CGI programs, but the following two technologies are more successful.

1)Start back-end batch mapping software with CGI, the feature of this software is that users can directly input instructions one line at a computer terminal. Its characteristic is that every requirement of users should start corresponding GIS software, if the software is large, the start-up time will be long.

2)CGI starts back-end Windows GIS software, the information exchange between CGI and back-end GIS software is accomplished through IPC-Inter Process Communication, the commonly used IPCs are RPC (Remote Procedure Call) and DDE (Dynamic Data Exchange). Its advantage is that because the GIS software is message-driven, CGI can only drive the GIS software to perform specific operations by sending messages, and does not need to restart every time.

Server API method

Server API is similar to CGI. The difference is that CGI programs can run independently, Server APIs are often attached to specific Web servers, such as Microsoft ISAPI attached to IIS (Internet Information Server), which can only run on Windows platform, and their portability is poor. But the Server API will always be running after it is started, and its speed is faster than CGI.

Plug-in method

With CGI or Server API, although the client’s interactivity is enhanced, the information the user gets is still static. Users cannot manipulate a single geographic entity and quickly zoom the map because at the client end, the entire map is an entity, and any GIS operations such as zooming in, zooming out, roaming, etc. require the server to complete and return the results. When the network traffic is high, the system response is slow. One way to solve this problem is to use plug-in technology, which is software that can exchange information with a browser, third-party software developers can develop plug-ins to enable browsers to support data files in their specific format. With the browser plug-in, you can transfer the functions of some servers to the client, in addition, for WebGIS, the plug-in processes and transmits the vector format spatial data, and the amount of data is small, which speeds up the response speed of the user operation and reduces Network traffic and server load, the downside of the plugin is that, like traditional application software, it needs to be installed before it can be used, which is inconvenient for use.

The following is an example of using WebGIS plug-in to display spatial data on the Internet, the plugin makes it easy to roam, zoom, and query maps.

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WebGIS plug-in. Figure is from Digital Earth Studio, Peking University (HTTP://WWW.CYBERGIS.ORG.CN)

Java Applet (Java Small Application) method

The WebGIS plugin can effectively process spatial data together with the browser, but its obvious shortcoming is that the calculation is concentrated on the client, called “fat client”, while for the CGI method and the Server API method, the data processing is performed on the server side. Form a “thin client.” Java language can make up for the shortcomings of many traditional methods, Java language is an object-oriented language, its greatest advantage is SUN’s slogan “Write once, run anywhere.” It refers to its cross-platform characteristics, in addition, Java language itself supports exceptional processing, network, multi-threading and other features, and its reliability, security makes it an important programming language on the Internet.

After the Java language is compiled, it generates platform-independent bytecode (Bytecode) that can be interpreted and executed by the Java Virtual Machine (JVM-Java Virtual Machine) on different platforms. There are two kinds of Java programs, one can run independently, and the other is called Java Applet, which can only be embedded in an HTML file and interpreted by the browser. The implementation of WebGIS with Java Applet is better than the plug-in method: 1) At runtime, Applet downloads from the server and does not require software installation; 2) Because the Java language itself supports network functions, it can realize the direct connection between the applet and the server program, so that the data processing operation can be implemented on the server or on the client side to achieve the balance of the load at both ends. Figure 13-16 shows the structure of a WebGIS system implemented using Java Applet.

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WebGIS system framework implemented with Java Applet

ActiveX method

Another technology that can implement WebGIS is ActiveX, which is a new Internet technology developed on the basis of Microsoft’s OLE technology, its foundation is DCOM (Distributed Component Object Model), which is not a computer language, but a technical standard. A component developed based on this standard is called an ActiveX control and can be embedded in an HTML file like a Java Applet and run on the Internet. Compared with the Java Applet, the disadvantage is that it can only run on the MS-Windows platform, and because it can perform disk operations, its security is poor, but the advantage is that the execution speed is fast, and since the ActiveX control can be implemented in multiple languages, in this way, the source code of the original GIS software can be reused, and the software development efficiency is improved.

Several WebGIS implementation schemes are described above. In the actual system construction, different schemes can be selected according to the data quantity, data type, Web server software, client requirements and so on.

Table 13-7: Several important foreign WebGIS technical features [Song Guanfu, etc.]

MapInfo ProServer

GeoMedia Web Map

Internet Map Server(IMS)

MapGuide

ModelServer/

Discovery

Company

MapInfo Corp.

Intergraph Corp.

ESRI Inc.

Autodesk Inc.

Bently

Server operating system

Windows NT/95

Windows NT

Windows NT

Windows NT

Windows NT

Web server

Web Server Supporting CGI

Internet Information Server

Internet Information Server or Netscape Server

Web Server Supporting CGI

Netscape Server

Other server-side software

ODBC,

MapInfo 4.x,

MapBasic

ODBC

ArcView or MapObjects applications,

ODBC

ODBC

MicroStation

GeoGraphics

ODBC

Client Operating System

Windows Series, Macintosh, UNIX

Windows NT/95

Windows Series, Macintosh, UNIX

Windows NT/95

Windows Series, Macintosh, UNIX

Client Browser

Any browser that supports HTML

Internet Explorer,

Netscape Navigator

Any browser that supports HTML

Internet Explorer, Netscape Navigator

Internet Explorer,

Netscape Navigator

Does the client need plug-in/control?

Unwanted

If you use Netscape Navigator browser, you need to install ActiveCGM plug-in; if you use Internet Explorer browser, you will automatically download ActiveCGM control.

Automatically download Java Applets or ActiveX controls

You need to install the MapGuide plug-in (about 1 trillion)

VRML, CGM, SVF plug-ins need to be installed

Graphic format for network delivery

JPEG (raster chart)

ActiveCGM (raster and vector)

JPEG/GIF (raster chart)

MWF (Vector Graph)

JPEG, PNG, VRML, CGM, SVF (raster and vector graphs)

Map Pre-publishing processing

Dynamic mapping

Dynamic mapping

Dynamic mapping

Pre-publishing of maps

Dynamic mapping

Publishable data format

MapInfo map file

MGE Engineering, MicroStation DGN Files, FRAME Files, MGEDM Files, ArcView Shape Files, Arc/Info Coverage, SDO Files

ArcView Shape File, Arc/Info Coverage, SDE Map File, Autodesk DWG File

Autodesk DWG

GeoGraphics Engineering Document, MicroStation Design Document </TBODY>

At present, the WWW field, Extensible Markup Language (XML) has received more and more attention, it can become a “meta-language”, used to define a specific domain of markup language, also in the Internet of spatial information. In publishing, XML can also be used to define specific language tags for geographic information to format and transfer data in an easy and consistent manner.