[Reprint] XML ?? The solution to the Development of WebGIS.

[Author: Zhu Weining Huang Xingyuan Ma Jinsong posted from: Nanjing University hits: 247 updated: Article entry: Ken]

Abstract: This paper discusses the technical difficulties faced by WebGIS and the concepts and features of XML, a new Web language. It aims to illustrate the advantages and good development prospects of XML application in WebGIS, taking the three-layer architecture solution of WebGIS as an example, this paper briefly describes the application of XML in WebGIS.

Keywords XML (eXtensible mchp language) WebGIS Geographic Information System (GIS)

1 Introduction

With the rapid popularization of Internet applications and the increasing development of technologies, especially the rise of international "Digital Earth" research and the proposal of the "National Spatial Data Infrastructure" solution, originally, GIS Based on the clien/Server structure is facing a wealth of new spaces with unlimited potential. Following this trend, the webgi must be the development direction in which GIS can fully utilize its capabilities and provide more effective services. Based on the existing intemedln133 architecture, the Web Service Based on Browser/servers can make full use of a large number of web resources to achieve seamless integration, providing more extensive geospatial information services to a wider range of GIS users has become the main trend of international GIS development. XML, especially for existing network deficiencies, has been widely accepted by major GIS software providers, including ESRI, inergraph, and Maplnfo, it is being tested as a key technology of the new generation of GIS. The use of XML in WebGIS is related to some technical difficulties faced by Internet and WebGIS.

2. Technical difficulties of WebGIS and XML proposal

WebGIS is a geographic information system that uses Word Wide Web to provide geospatial information services to various types of users. It is a product of the combination of Internet and GIS. However, intenet is not a dedicated GIS network, and its initial application is not based on the Internet. Therefore, it is difficult to combine new and old things.

2.1 WebGIS technical difficulties

The traditional Web language is widely used in HTML hypetext Markup Language (hypertext mark language). Its essence is a text display language. With the increasing number of Web Information types, the disadvantages of displaying geographical space data are gradually exposed. Therefore, the technical difficulties faced by WebGIS are not easy to solve:

2.1.1 because HTML pages are only good at data performance and lack the internal structure and connection of descriptive data, it is not conducive to the query and integration of spatial geographic information data with complex structures.

2.1.2 the data that makes up the digital earth will be maintained by thousands of different organizations. It is necessary to adapt to the high-efficiency and low-cost conversion of Web expressions for a large amount of geographic information data in traditional GIS databases, each WebGIS requires the sharing of resources and information, so that GIS data is physically dispersed and logically concentrated.

2.1.3 according to the requirements of the digital earth, WebGIS requires a certain level of interoperability, so that GIS data is involved in various applications. However, once HTML pages are generated, the information is static, dynamic changes cannot be made based on the actual situation of the client.

2.1.4 because GIS processes massive data and is subject to Internet bandwidth and other route restrictions, a fast response and transmission mechanism should be established to meet the needs of user interaction operations, provides rapid Geographic Information Services to WebGIS users.

2.1.5 WebGIS needs to provide users with a variety of intuitive and easy-to-understand graphical user interfaces, predict customer requests, and dynamically and customizes data.

In the face of the above technical difficulties and the trend of international information integration, XML will be a solution for WebGIS to adapt to the development and requirements of the digital earth and improve its practical application capabilities.

2.2 XML overview and features

XML (Extensible Markup Language) is a new Web language developed by W3C (word Wde Web Consortium) to adapt to the development of the Internet and solve the above technical difficulties, it is a simplified set of SGML (Standard Generalized Markup Language, general language identification standard) developed by ISO. It is not a predefined HTML-like identity language, but a metalanguage used to define other identity languages. Different from a fixed number of tags in HTML, various tags used to describe information in XML can be created by the designer to strengthen the structure and Association of specific professional data.

The introduction of XML in WebGIS has great advantages and functions:

2.2.l helps standardize and structure geospatial data. Geographic Data can be uniquely identified by XML to facilitate online query and search, facilitate information sharing in Digital Earth resources, and improve the interoperability of WebGIS services, this reduces the frequent interaction between servers and customers and increases the interoperability speed of GIS users.

2.2.2 XML has the diversity of data sources and the flexibility, flexibility and adaptability of multiple applications. XML can combine and integrate structured GIS data from different sources. After obtaining XML data, the customer can develop various forms of wallgis application software, it can also be used for local geographic computing and secondary processing, such as measurement, drawing, spatial analysis, and geographic modeling, to expand the application of XML and GIS data in multiple aspects.

2.2.3 due to the separation of content and form, XML only describes GIS data. The specific expression of data can be converted using the style sheet language, the geographical information can be dynamically displayed based on the customer's configuration and actual conditions.

2.2.4 it is feasible to use XML to transmit GIS data on the existing web. It does not need to change the network base and uses the original HTTP protocol, resulting in a low cost.

2.2.5 XML provides open standards and support from many software companies. The xml1.0 version developed by W3C has been released, and the W3C provides unified standards for languages and interfaces related to XML processing. Microsoft, Netscape, and many international database software companies are already providing XML support and services. OGC also developed an XML-based language for WebGIS: GML (Geography Markup Language, geographic identification language ).

3 Application of XML in WebGIS

XML is used to analyze and describe data content and structure. Therefore, XML can be applied to any WebGIS solution in principle. Due to the powerful functions of XML, its implementation and exploitation process is more complex than that of HTML. Taking the establishment of a common wibgis layer-3 service solution as an example, we will briefly describe the specific application of XML.

The three-layer service structure is a WebGIS Solution Based on GIS basic database, intermediate layer, and Web client browser. Regardless of the implementation of these three layers, XML can play an important role in data storage, exchange and performance.
3.1 basic Server:

The Geographic Information Server and basic database at the bottom are the data sources of WebGIS, which store a large amount of raw non-xml gis data. These data are either fully converted and stored as XML data, either maintain the original data form and convert it to XML data based on customer requests through the middle layer for Web use. With the increasing support for XML by many database providers, mainstream databases in the future can directly store and interactively query XML data. Pay attention to the following aspects when using XML data on the Data Server:

Ensure the structure and validity of the XML file. Strictly write and exchange XML files according to W3C XML standards and OGC specifications, and use DTD (Document Type Definition, file type definition) or schema description and definition of all identifiers used in XML, so that the processor can learn the source of XML and the spatial data types, attributes, and relationships among other related geographical information.

Maintain the consistency of GIS data formats. Although XML allows designers to define their own XML identifiers, maintaining the consistency of WebGIS-XML definitions is conducive to data processing and interaction. Taking the GML language of OGC as an example, it defines the relationship between simple geometric feature sets based on points, lines, and polygon and the three SRS associated with them (spatial reference system, the DTD definition of the spatial reference system. The following is the definition of the geometric set:

The bottom layer of geometric features is The spatial coordinate point set marked. The subsequent extended set uses the element value of XML to represent the feature value of the feature set, the attribute values of XML elements represent the names of feature sets and SRS sets and non-spatial attributes of geographical information. The XML Representation of a polygon is shown below:

Although GML is still in the draft phase recommended by OGC, it is wise to use this as a reference for every WebGIS.

3.2 middle layer:

The middle layer of WebGIS is used to respond to client requests and identify and convert XML data. It applies for spatial and attribute data from the underlying database and directly communicates with users. If the vector and grid data of GIS is applied for from the underlying database, it needs to be converted to a standard XML format. If the underlying database supports XML storage and query, the middle layer must submit a data application sent from the client to the underlying layer, summarize and sort the database response data, and uniformly schedule and allocate data to the client browser.

For data stored in XML format, if the database does not support XML, you can directly query XML data in the middle layer. What is the XML query language recommended by W3C ?? XML-QL, its return value is flexible and diverse features, can return XML results tree and charts, directly submitted to the customer.

Dom (Document Object Model, Document Object Model) can be used to provide a special program for WebGIS to access GIS data in XML documents. These methods are implemented through the XML parser, it provides an interface for each WebGIS service provider to access procedural XML data. Many Resolvers, including Microsoft and Netscape Resolvers, provide Dom functions. The W3C Dom Level 1 standard defines how the DOM structure implements attributes, methods, and events. Taking Microsoft VBScript and IE as an example, you only need to create an instance of the parser through MSXML. dll in the program:

Set objwebgisparser = Createobject ("Microsoft. xmldom"

You can access and process XML geographic information nodes through the included Type Library and code, access XML documents, and obtain node feature values, data types, and spatial attributes.

The middle layer also needs to provide the name space service to the XML file to resolve the conflict of identities of different XML files in specific applications. For example, when XML files from different WebGIS service providers are merged, you can use its own web-URL path and the address of the shared DTD file as the uniform identifier prefix to ensure the uniqueness of the identifier.

3.3 client:

The WebGIS Service provides personalized and diversified display of response data on the client based on the customer's actual network situation. XML provides this capability. XML parser is used to display XML data. Microsoft ie5, Netscape Navigator 6.0, and other browsers can parse XML. With the development of XML, more network applications support xml.

Due to the separation of XML content and performance, the specific display form of XML on the WebGIS client is determined by style sheet. XSL (extesible Stylesheet Language, extensible Style Language) is a style table language of XML, used to convert XML into a variety of language pages that can be recognized by the network, such as HTML pages. Due to the graphic characteristics of GIS data, it can be converted into multiple web graphics languages (such as pgml, SVG, and VML) in WebGIS applications ), some of these conversion engines will also emerge one after another. XSL also facilitates the customer's interactive query and shortens the response time. For example, when Tourists Seek the shortest path and the most economical path to the destination, XSL can search and sort the same XML file to provide the result list for the customer, and instantly displayed on the map.

XML link language (Xll) improves HTML's hypertext simple link and provides more powerful functions. It adds optional links, supports scalable links and multi-direction links, and supports domain names, bidirectional links, loops, and so on independent of addresses, make full use of WebGIS resources.

WebGIS can make full use of all the functions of XML in dynamic data display. Microsoft recently released an XSL processor in the form of ActiveX to process XML files in the browser. Creating a Java-based virtual machine on the client is no longer the only option, A considerable proportion of data can be processed by the client through XML, and the data does not depend on the platform, language, and other restrictions, even in a potential market of WebGIS ?? ?? It can also act as a key role.

4 Conclusions and prospects

Nanjing University has started some basic research work on the application of WebGIS and XML, and is committed to integrating the GIS data in China with the requirements of XML standardization and standardization. The combination of WebGIS and XML has obvious advantages. It is particularly significant to adapt to the trend of Digital Earth and the national condition of GIS in China. This is both a development direction and an opportunity and a bright future ~ Step by step to promote the development of China's GIS industry.

References

1 Huang Xingyuan. Introduction to geographic information system, Higher Education Press, 1989

2 Extensible Markup Language (XML) 1.0, www.w3.org/tr/rec-xml