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Appendix II. Technical Issues

A number of technical issues arise when it comes to delivering real-world archaeological and historical data across the web. Not least of these is the issue of download size. Many schools and home users use dial-up modem connections, and may well have to pay for traffic by the megabyte, so it is essential that downloads be kept to a small size and that repetitive downloads of the same data are avoided.

Datasets

Our China TimeMap dataset, which includes instance outlines for approximately 1500 dynasties, kingdoms and rebellions,  runs to around 5 Mbytes and hundreds of thousands of data points. The size of such datasets can be offset to some degree by data compression, by server-side data generalisation one does not require fine detail to look at a zoomed-out view of a complete dataset and by caching and serving data on-demand, rather than downloading complete datasets. In this way most datasets can be reduced to acceptable amounts of data, even for modem-based access.

Bitmap animations

A more serious problem is the size of animations. A 30-second map animation of the spread of the Ming runs to 0.5 Mbytes with good quality and compression - already large for web delivery over a modem connection - and the full animation of the China TimeMap data over approximately 5 minutes in a 300 x 200 window, runs to over 30 Mbytes, which is prohibitive. These large file sizes are based on chaining together sequences of bitmap images which, even with compression, take a lot of bandwidth.

Flash animations

Fortunately, Shockwave Flash format (.SWF) provides us with a potential solution. Flash supports layering of objects, so static background maps can be stored as a single object. Flash plugins provide support for tweening vector objects (morphing one outline into another) so that intermediate frames need not be stored, making for smoother animations and much reduced data requirements. Flash animations are also more inherently scaleable both in size and duration without increase in data volume. We have used TimeMap and Flash to create a high-resolution 10-minute animation of the history of the Shilla Dynasty in Korea, which can easily be scaled to 1600 x 1200 resolution, yet the animation totals just 1.3 MByte and streams so that it starts playing as soon as enough data are received.

Browser independence

Java portability, compatibility, stability and long-term support are also issues for complex web mapping applications such as TMJava. In our experience, compatibility between different browsers, particularly Netscape and Internet Explorer, can only be achieved using the Java 1.2 plugin, and this is not available for Macintosh. Furthermore, there are some issues with long-term support for Java in the face of Microsoft market dominance and their sponsorship of alternative approaches to web services. As a result, any web-based mapping application will need to evolve perhaps involving substantial rewriting of some components if it is to remain competitive, or even workable, in an ever-evolving web environment.


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Last updated: Wed Sep 11 2002

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