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1、The potential of XML encoding in geomatics converting raster images to XML and SVGByron Antoniou, Lysandros TsoulosAbstractThe evolution of open standards and especially those pertaining to the family of XML technologies, have a considerable impact on the way the Geomatics community addresses the ac
2、quisition, storage, analysis and display of spatial data. The most recent version of the GML specification enables the merging of vector and raster data into a single open format. The notion of coverage as described in GML 3.0 can be the equivalent of a raster multi-band dataset. In addition, vector
3、 data storage is also described in detail through the GML Schemas and XML itself can store the values of a raster dataset, as values of a multi-table dataset. Under these circumstances an issue that must be addressed is the transformation of raster data into XML format and their subsequent visualiza
4、tion through SVG. The objective of this paper is to give an overview of the steps that can be followed in order to embody open standards and XML technologies in the raster domain. The last part of the work refers to a case study that suggests a step by step methodology to accomplish classification,
5、an important function in Cartography and Remote Sensing, using the XMLencoded images. 2005 Elsevier Ltd. All rights reserved.Keywords: Open standards; Raster image encoding; Classification1.IntroductionThe advent of XML-based technologies has gone beyond the expectations of the most optimistic users
6、. Revolutionary ideas are emerging for the storage, exchange and display of data and new formats are created for almost all kinds of data, applications and knowledge domains. A considerable number of specifications have been issued by international organizations aiming at the provision of an efficie
7、nt open environment to the user community. Watching this frenzy trend of transforming everything into XML-based structures, one thing seems really out of the way: raster images. Languages like HTML and SVG do not have such a structural feature in their environment. Instead, both of them provide mean
8、s (i.e., or elements) for the incorporation of raster images in text-based files, usually with an inline reference. The existing specifications instead of dealing with the problem bypass it and users treat raster formats more or less like a requisite tool for their work. The truth is that there is n
9、othing common between XMLbased structures and raster images. Raster image encoding is neither text based nor human readable and it cannot be parsed, checked for validity or well formedness. Moreover, the raster image content has almost no flexibility (apart from resizing) in an XMLbased environment,
10、 since pixel values are well locked inside the raster formats (Antoniou and Tsoulos, 2004).Converting a raster formatted image into XML enables the user to utilize the information residing in the image and select, read and manipulate the parts of the XML file or the file as a whole in a number of wa
11、ys in accordance with the application at hand. This is the starting point for classification, statistical processing, filtering or the development of other applications with the use of XML technology. Operations like the storage and exchange of images acquire a new meaning in the framework of an int
12、eroperable environment like WebGIS. Converting an image from raster to SVG is even more exciting. An SVG-encoded image file enjoys all the above-mentioned advantages; in addition the user can visualize the effect of every change imposed on it. Geomatics is a sector that depends on images to such an
13、extent that one could tell that images are the most valuable geo-data sources. Feature extraction from a raster image is a very common task for geographical organizations around the world. Through the combination of image information and SVG code instead of vector data, digitization can produce scal
14、able vector data (i.e., , , , etc. elements) or GML (Geography Markup Language)- encoded data. Moreover, the need for visualization of the new components introduced by GML 3.0 specification, such as Grid functions, requires the rendering of continuous data in XML (Antoniou and Tsoulos, 2004).Another
15、 area that will be influenced due to the evolution of open standards is interoperability. Provided that each satellite sensor stores data in its own format, the same applies to proprietary software, which also uses closed formats. The efficiency of remote sensing applications and interoperability ca
16、n be enhanced with the use of open standards. By storing a raster dataset in an XML-encoded format, the information included in the dataset is conveyed intact along with the advantages that open standards bear. This paper elaborates on the technological environment, which can be utilized for the XML
17、 encoding of raster images and the extraction of information from the resulting datasets using open source methods.2. Technological background2.1. Extensible markup languageXMLXML stands for Extensible Markup Language and it is a W3C-endorsed standard for document markup. XML describes a class of da
18、ta objects called XML documents and partially describes the behavior of computer programs that process them. XML documents are made up of storage units called entities, which contain either parsed or unparsed data. Parsed data are made up of characters forming character data or markup. Markup encode
19、s the description of the documents storage layout and logical structure. Furthermore XML provides a mechanism to impose constraints on the storage layout and logical structure (Bray et al., 2000).XML is a meta-markup language implying that it enables the user to create his/her own tags according to
20、the applications needs. That means that XML does not have a fixed set of tags and elements that cover the needs of every application or user. Markup in an XML document describes the structure of the document along with the documents semantics. XML allows the developers to define properly the element
21、s required and to encode their associations. XML defines the syntax that markup languages of each knowledge domain, such as MusicML, MathML, GML and SVG must follow. Although it is quite flexible in the elements definition, it is rather strict in many other aspects. It provides grammar rules for the
22、 XML documents describing their proper structure, which allows for the development of XML parsers that can read any XML document. Documents that satisfy this grammar are considered as well formed (Elliotte and Means, 2002).2.2. Geography markup languageGMLGML is a markup language used to encode and
23、integrate spatial information, spatial relationships and non-spatial information, especially when non-spatial data are XML-encoded. GML also aims to serve both data transport and data storage, in a wide-area Internet context. GML exploits W3C standards to encode geographic information that can be re
24、adily shared in the Internet. In addition, GML provides a set of common geographic modeling objects to enable interoperability of independently developed applications. It is designed to support interoperability and does so by providing basic geometry tags, a common data model and a mechanism for cre
25、ating and sharing application schemas. Although it is not the first meta-language introduced to describe geographic information, it is thefirst that has been widely accepted by the GIS community. An important characteristic of GML inherited from the XML specification is that it separates spatial and
26、 non-spatial content from presentation (Galdos, 2003).2.3. Scalable vector graphicsSVGSVG stands for Scalable Vector Graphics, an XML grammar for stylable graphics used as an XML namespace. SVG is a language for the description of twodimensional graphics in XML and allows for the encoding of three t
27、ypes of objects: vector graphics, images and text. Graphic objects can be grouped, styled, transformed and composed into previously rendered objects. The feature set includes nested transformations, clipping paths, alpha masks, filter effects and template objects, which are applied during rendering.
28、In general, SVG drawings can be interactive and dynamic. Animations can be defined and triggered either declaratively or via scripting (Ferraiolo, 2001). SVG graphics are scalable to different display resolutions. The same SVG graphic can be displayed at different sizes on the same Web page and re-u
29、sed at different sizes on different pages. SVG graphics are scalable because the same SVG content can be a stand-alone graphic or can be referenced or included in other SVG graphics, thereby allowing a complex illustration to be built up in parts, perhaps by several people. During rendering SVG also
30、 provides client-side raster filter effects so that moving to a vector format does not result to the loss of popular effects such as soft drop shadows (Ferraiolo, 2001).Other characteristics of SVG include a smaller file size and searchable text information. An SVG fileutilizing the elements provide
31、d by the specificationis usually smaller than a raster file for the same map resolution and thus can be transferred across the Internet more quickly. Text information inside SVG is still text and can be searchable, while text information inside the raster file becomes integrated into the image and i
32、s no longer recognized as text. SVG is also particularly suitable for displaying intelligent maps, because geometric objects such as points, lines, and polygons are recognized as such and are identifiable.Raster images on the other hand contain information about every pixel, and points, lines and po
33、lygons that are no longer recognizable. Therefore, the user can directly work with spatial features on an SVG but not on a raster graphic image (Peng and Zhang, 2004).SVG is also based on XML and therefore conforms to other XML-based standards and technologies, such as XML Namespace, XLink, and XPoi
34、nter. XLink and XPointer allow for linking from within SVG files to other files on the Web, like a GML data element, HTML pages or other SVG files (Boye, 1999).2.4. Extensible stylesheet language transformationXSLTXSLT is a language that enables the user to convert XML documents into other XML docum
35、ents or into almost any form an application or a user needs. XSLT provides an easy, W3C sanctioned, way to convert XML documents that conform to a schema into documents that conform to another, enabling the sharing of information between different systems. From another perspective XSLT is a programm
36、ing language that describes the way and the methods to be followed for the transformation of a well-formed tree structure of an XML document to another. XSLT is not the only way to achieve these goals; there are alternative ways to transform XML documents but XSLT has prevailed. The fact that XSLT i
37、s a W3C standard implies that XSLT complies with the specifications published by the W3C or those that will be announced in the future (Clark, 1999). Furthermore, the non-proprietary status of the specification and the platform independency, guarantee the prospects and the integrity of the specifica
38、tion. In addition, the fact that the XSLT document transformation instructions are stored as an XML document is an advantage since there is no need for using another syntax. The above-mentioned characteristics justify the popularity of XSLT when it comes to XML transformation (DuCharme, 2001).2.5. D
39、ata model encodingThe accurate representation of the complexity of the real world has been one of the fundamental problems in Geomatics. In order to address this issue the GIS community has introduced a number of ways for modeling real world data.In a vector model, reality is perceived as an aggrega
40、tion of discrete entities defined by their geometry,topology and thematic attributes. Another popular way to describe real world phenomena is by using raster structures that can sufficiently describe continuous/field data. The raster model in its basic form is considered as a rectangular array of eq
41、ually spaced pixels. Each pixel can be defined uniquely inside the array through i,j coordinates. The values stored in the pixels of the array depend on the phenomenon and can be from elevation or temperature to the reflectance of light for part of thespectrum.2.5.1. The XML approachXML enables doma
42、in experts to create properly structured formats, which can serve the storage and exchange of a wide variety of data types. Apart from that XML itself can store efficiently various types of data and it can easily describe continuous data using a table-based mapping. The table-based mapping is used t
43、o model XML documents as a single table or set of tables (Fig. 1).Fig. 1. Table-based mapping of field data.This general form can be modified accordingly in order to accommodate the description of different phenomena. For instance the description of a threeband raster image could be achieved in the
44、way shown in Fig. 2. A problem easily solved through XML is whether the auxiliary data should be stored as child elements or as attributes, as well as the names to be used for each element or attribute. In addition, developers who use table-based mappings often include table and column metadata eith
45、er at the beginning of the document or as attributes of each table or column element. For image metadata one could store pixel size, date and conditions of capture, details of the camera used, etc. Using this method efficient description, storage and exchange of field-type geographic data can be ach
46、ieved utilizing open standards and promoting interoperability (Bourret, 2004).2011718120317578.Fig. 2. Description of a three-band raster image.Apart from continuous data, XML can easily encode data modeled as vectors. An XML schema is developed describing the definitions of geometric primitives. In
47、 fact that is what OGC provides through GML, which is an XML-based specification designed to describe vector data.2.5.2. The GML approachGML has been a turning point in Geomatics to the extent that many national and private organizations have already adopted this format. GML 3.0 specification introduced a number of new components improving vector data encoding (Cox et al., 2003). GML 2.1 provides only three core schemas (features.xsd, geometry. xsd and xlinks.xsd) whereas in GML 3.0 there are 28 core schemas.This version supports new geometry types includ