The Hyper-Layering

This page is English Version of the Japanese article of this blog.

The most important function of SVG Map as the Web Map platform is the Hyper-Layering.
This function was standardized as SVG1.1 of W3C. However, the arranged explanation concerning this function is not performed in the SVG1.1 specifications. Then, to clarify the behavior of the function, this chapter explains the Hyper-Layering function.

Introduction:
Hyper-Layering is a function to succeed to the basic philosophy of WWW. Therefore, let's reconfirm WWW for the understanding of its meaning.

World Wide Web is an information platform designed under the basic idea that the Internet can be used as a huge database by connecting the information stored computers in over the world (World Wide) connected to the Internet in the shape of a cobweb (Web).

To achieve this, the following unique system architecture was given to WWW.
It offers the cooperation characteristics (or interoperability) that had high versatility with simplicity by giving priority to human understandability in substitution for machine processibility. And, HTML, HyperText and URL realizes it.

1. HTML : Document expression information for person to understand
   It is a document information with the format. It can put graphics data ancillary.
2. Hyper text : Information connection method by user interface
   It is a text where the hyperlink is set up. However, the meaning to which the machine can be understood is not given to the hyperlink.
3. URL+HTTP : Simple access means of each document
   It doesn't use an advanced query language like DBMS, and be an access means to the file on the Internet.

Purpose of Hyper-Layering:
The document which conventional WWW handled was letter information with a format. On the other hand, main document of geographic information is "Map" to abstract geographical features by figure. Moreover, the information connection means that conventional WWW used was HyperText. On the other hand, technique to understand connection of the information by making a comprehensive survey by plural information composed as a "map" is used for geographic information. It is technique to be called "layering". Thus, the method of an expression and cooperation unlike conventional WWW had been adopted for geograohic information from its peculiar characteristics.

Then, it is the Hyper-Layering to enable the use of the technique used by such a geographic information by enhancing WWW. Hyper-Layering connects the geographic informations by the layering of two or more maps indicated by the hyperlink.

Specification of Hyper-Layering:
Hyper-Layering is expressed by the SVG data. Its meaning is that an SVG format takes a function of inter-operation role (the Hyper-Layering) as well as expression of a map (map expression by figures). By the way, the HTML takes document expression (a formatted document) and inter-operation (HyperText), too.

Hyper-Layering is basically executed on the browser in accordance with SVG Map Profile by the following procedures.

1. A SVG data (which called container) that directs Hyper-Layering is read.
2. It accesses the map layers according to the hyperlink described in the container.
3. The acquired map layer data is synthesized (layered), and the desirable map presentation is obtained.

These processing can be executed by the browser, and open standardization of this procedure for WWW is promoted. That is a reason why we say that only SVG Map is Web Map. And there are many map services that it is not appropriate to call "Web Map" as follows.
* The cooperation of geographic information by the hyperlink cannot be done.
* Even if it is possible to cooperate, the complicated procedure different from a mere hyperlink is used.
* Or, it can be executed only on the server side.
* Open standadization is not promoted. (It tends to use an exclusive browser by Javascript etc.)
In other words they are map service which merely borrowed communication environment of the Web. And, these might promote making of sterilization of WWW. (See also:http://yupnet.org/zittrain/)

image : XML element that executes Hyper-Layering
An image element is an element prescribed by SVG1.0 and offers a function to stick other graphics documents ordered in the hyperlink attribute (xlink:href) on an SVG document. Therefore Hyper-Layering is carried out by image element by appointing a document to compose. We call SVG data with hyperlink to the map layer to carry out hyper layering the "container".
In addition, the order of composition (layers) obeys the drawing order of SVG documents. That is, the element described before than the image element is piled below, and the element described later than the image element is piled up.

Geographical coordinates : Required meta data for Hyper-Layering
The individual SVG document has an individual coordinate system, but they have to be composed while merging the geographical position. Therefore geographical coordinate must be shown in each SVG map document. The specifications about the setting of the geographical coordinate are mentioned here.

width, height attributes of image element : Attributes that is omissible for Hyper-Layering
With the normal SVG specifications, we cannot omit the width,height attribute of the image element. On the other hand, the first purpose of the Hyper-Layering is to layer a map definitely. It may be carried out an SVG map having geographical coordinate meta data without those attributes. Therefore it is assumed that SVG Map Profile can omit these attributes so that hyper layering is more easily used. In addition, a new function will be carried out when these attributes were described.

Example:
<svg  xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:go="http://purl.org/svgmap/" viewBox="122.93 -45.52 20 20" >
<metadata>
<rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#" xmlns:crs="http://www.ogc.org/crs" xmlns:svg="http://www.w3.org/svg" >
  <rdf:Description>
   <crs:CoordinateReferenceSystem rdf:resource="http://purl.org/crs/84"  svg:transform="matrix(1.0,0.0,0.0,-1.0,0.0,0.0)" />
  </rdf:Description>
</rdf:RDF>
</metadata>
<image xlink:href="http://www.svg-map.org/svg/basemaps/cyberJpN35.svg"/>
<image xlink:href="http://www.svg-map.org/wms/smap.svg?dataset=bosai"/>
</svg>