Scrolling is one of the most common tasks performed when writing, editing or browsing a document. Because it is done so often, even a small improvement can result in a significant overall saving in time. Thus, if it is not optimised then a large amount of time and money is being wasted on this mundane task. The standard techniques used in document navigation are a scrollbar, panning, or ---more recently--- rate-based scrolling. However, these techniques suffer from some common limitations.

It is becoming increasingly common for users to work with large documents. The increasing power of computers and global exchange of data has resulted in it being both possible and practical for large documents to be written and exchanged in electronic form. The size of such documents might be 20, 50 or even a 1000 pages. Documents of this size make increasingly important the question of navigation. However, it is exactly with these larger documents that standard techniques perform increasingly poorly. Panning is impractical over any large distances. A large document can mean that moving the scroll-thumb one pixel jumps the document by a significant amount, meaning that scrolling is not smooth and that the target may be skipped. A scroll bar also forces the user to move back and forth between the document and the scroll bar which is unnecessary overhead and can result in a loss of focus. Rate-based scrolling is an alternative approach where the displacement of the mouse from the point where scrolling began is mapped to the speed of scrolling. Thus, the further the mouse is moved when in scrolling mode, the faster the document scrolls. This method provides successfully for smooth scrolling within large documents. However, rate-based scrolling suffers from a limitation on its speed. Attempting to scroll very rapidly means that the information becomes too blurred. This imposes an upper-limit on the speed of scrolling.

A solution to this problem which attempts to retain all the advantages of the rate-based system is Speed-dependent Automatic Zooming (SDAZ). The concept of SDAZ was first investigated by \citeasnoun{ih:1} who suggested that the solution to the blurring-problem was for the system to invoke an automatic zoom-out once the scrolling speed exceeded a predefined velocity. The zoom-out reduces the problem of blurring as the document may be scrolled through more rapidly without the information moving across the screen as fast. SDAZ thus retains the advantages of rate-based scrolling while removing the problems associated with rapid scrolling, hence allowing the rapid acquisition of targets that are a great distance away in the document.

\citename{ih:1} performed a preliminary evaluation of SDAZ on several interfaces (Web Browser, Map Viewer, Image Browser, Dictionary, Sound Editor) with mixed results. \citeasnoun{cs:1} performed a more rigorous evaluation of SDAZ in a Document Viewer and a Map Viewer and obtained very positive results. This paper focuses on the use of SDAZ in viewing documents. The issues surrounding the implementation of SDAZ systems are discussed and an SDAZ system is evaluated formally against rate-based scrolling and standard scrolling.

Previous work on SDAZ and related topics is described in Chapter 2. In Chapter 3 we discuss the theoretical ideas that surround SDAZ. We examine the issues surrounding the calibration of SDAZ systems in Chapter 4 and we attempt to obtain an optimal implementation through user feedback. Chapter 5 describes a formal evaluation of SDAZ against rate-based scrolling and normal scrolling. Results, suggestions for future work, and our Conclusions follow in Chapters 6,7 and 8.
