Introduction
The MRF SVG Map Server is an SVG-based Map Server developed by MRF Geosystems.
Scalable Vector Graphics (SVG) is the emerging standard for the display of two-dimensional vector graphics on the Web. It has been developed by the World Wide Web Consortium (W3C) – the body responsible for Web standards such as HTML. SVG displays three types of graphics: vector graphic shapes, such as lines, arcs and polygons (including polygons with holes), text, and raster images, which can be positioned relative to other features in the image.
SVG is a grammar in XML (eXtensible Markup Language), another W3C specification. Through the use of a scripting language (such as JavaScript), and access to the SVG Document Object Model (DOM), images can be made both interactive and dynamic within the Web browser.
An SVG image can be made interactive through the assignment of event handlers, such as “onclick” and “onmouseover”, to image elements. These might be used, for example, to highlight a selected feature, show attribute information, or display a “map tip”.
Through the DOM, image elements and groups of elements may dynamically be given new styles (color, etc), made visible or not, moved or modified. New elements may even be added. All this can take place on the client machine – without a “round trip” to the server.
SVG is an ideal candidate for displaying GIS (Geographic Information Systems) information in a Web browser. All typical GIS features can be readily displayed (and manipulated), and data is transferred in a compact vector (rather than raster) format. Since SVG is XML, it leverages and integrates with other W3C standardization efforts. This means that facilities already developed, such as Cascading Style Sheets (CSS2) and Extensible Stylesheet Language Transformations (XSLT), can be readily employed – they didn’t have to be “reinvented” for SVG. Also, since it is XML, the SVG DOM plugs seamlessly into the DOM of browsers, such as those from Microsoft and Netscape, and is easily accessed from DHTML (Dynamic Hypertext Markup Language) pages. This facilitates easy to code, efficient, responsive interaction with the user.
Status
Technically, the W3C does not create standards – it publishes “Recommendations.” In the Web community, however, a W3C recommendation is as close to a standard as you’re likely to see. Currently, as of August 2, 2000, the SVG 1.0 specification has a status of “Candidate Recommendation.” This means that developers are encouraged to create implementations and provide technical feedback. The specification could be elevated to “Recommendation” status at any time.
SVG has the support of all the “big players”. These include Microsoft, Netscape, IBM, and Sun Microsystems. SVG is being promoted by the major graphics software vendors, such as Adobe and Corel. All of these companies (and about twenty others) have been active contributors to the standardization effort.
Once SVG becomes a Recommendation, it is expected to become a built-in supported document type for both Microsoft’s Internet Explorer and Netscape’s Navigator. In other words, SVG will be supported natively, and not require a “plug-in”.
In the meantime – until SVG is natively supported in the major browsers – plug-ins are available (at no cost) for rendering SVG images. The most popular of these is from Adobe.
Advantages of SVG
SVG is XML. As such, any tool used in support XML can be used in support of SVG. SVG can leverage other facilities developed to enhance XML – those facilities don’t have to be “re-invented” specifically for SVG. SVG will be the 2D vector graphics standard for the Web. Support should be broad and universal – not confined to the GIS or technical communities.
SVG is vectors. If produced intelligently, it can be more compact than equivalent raster images, and more compact than many vector formats, as well.
SVG will be built-in. Currently, a plug-in is required to view SVG in a browser, but this is likely to change shortly. Once SVG is supported natively by the major browsers, not only will it not be necessary to download and install a plug-in, you’ll never have to worry about upgrade problems when changing to a new version of a browser.
How MRF Is Using SVG
One of the acclaimed advantages of SVG is the ability to pan and zoom an image on the client side – since the image is based on vectors, it is “resolution independent”, and you don’t see the choppiness you would if you zoom in on a raster image. The problem with this as it relates to a GIS image server is that, as you zoom in, you expect to see greater detail. In order for that detail to be displayed, it would have to have been in the image sent to the client. More detail means longer transmission and rendering times. Panning presents a similar problem in that the image would need to contain information you would not initially be able to see. Carrying this to its logical conclusion, providing full panning and zooming on the client would entail downloading the complete feature sets from the server. Not a practical solution, in most cases.
The approach taken with the MRF SVG Map Server is to send only the amount of detail needed to render the image at the scale selected – panning and zooming is disabled. Several techniques are used to produce images that the orders of magnitude are smaller than they would be, if sent out at full resolution. Each pan or zoom requested by the user requires a trip back to the server for a new image, but the penalty is small, due to the reduced image size.
Data Security
The “minimum resolution” approach taken by MRF has a beneficial side-effect. Many data vendors are concerned that their spatial data-sets not be published on the Internet in such a way that they could be downloaded and put to unauthorized use. The MRF approach to SVG delivery discourages this in two ways:
- The points sent out are in pixel positions, rather than coordinates. When zoomed out, the resolution of these points will be very coarse (relative to the original coordinates), and likely to be of little use. When zoomed in, the point resolution will be better, but only a small portion of the data-set will be included in the image. In order to extract a significant amount of data with reasonably good point resolution, a large number of queries would need to be made, and then the data pieced together – a server could be programmed to detect this kind of activity.
- It is not necessary to include location, scale, or projection information in the image. Without this reference information, the data extracted would be less useful.
- Download plug in
Although we expect the situation to change in the next year, SVG is currently not supported natively in your browser. Adobe has the Version 2 SVG plug in available for free download at the following URL:
http://www.adobe.com/svg/viewer/install/
NETSCAPE / FIREFOX / SAFARI USERS: please note that the current version of the demo only operates on IE 5.0 and higher. Please contact us for information on when the SVG demo will operate in Netscape/Firefox/Safari.
2. Running SVG map server demo
Please download and install the SVG plug in before continuing with the online SVG demo at:
Some Useful Links
http://www.w3.org/Graphics/SVG/Overview.htm8
http://www.w3.org/TR/2000/CR-SVG-20001102/ (specification)
http://www.adobe.com/svg/ (good tutorial)
http://wdvl.com/Authoring/Languages/XML/SVG/ (more links)