\label{TaxChapter}
Some forms of document navigation are performed effectively by current methods, while others are not as well catered for. It is worth examining different types of document navigation in order to gain an understanding of the strengths and weaknesses in current methods of navigation and to obtain insight into just how the addition of SDAZ functionality would complement the standard techniques.

\section{Our Classification of Document Navigation}
\label{Tax}
We are not aware of any existing taxonomies of different navigation actions. We therefore propose the following characterisation of the different types of document navigation:

\begin{itemize}
\item Spatial-Locate

This is where the user knows where in their document their target is. For example ``I want to look at a page which I remember is about one-third of the way through this document.'' This is analogous to picking up a book and knowing approximately which section of the book you want to open it at.

\item End-Point

This is where it is desirable to navigate to the very start or very end of the document. This might often form the first part of a two part navigation action. For example, ``I want to go to the page that is two pages from the end of the document''.

\item Pagination

This is where the user wants of make minor adjustments to the information displayed on their screen. For example the user might be focusing on an item near the bottom of the screen and want to centre the screen on that location, or perhaps to look at the previous page.

\item Search-For

This is where the user remembers a certain phrase that occurs somewhere in the document and wish to navigate to that occurrence, or all occurrences thereof.

\item Page-Locate

This is where the user knows the page number of their target location. They may have a hard-copy open in front of them at the desired page, or they may have a reference to a page number in the document.

\item Reacquire

This is where the user wishes to return to their last location. An example of this would be the ``Back'' button on a web-browser. In document navigation this action would occur if the user was editing a section of a document and wanted to quickly check another relevant section before jumping back to the section they were editing.

\item Zooming

This is where the user wishes to change the magnification of the document, either to zoom-in and view it in finer detail in order to read small print or a detailed diagram, or to zoom-out to obtain a more general overview, or to return to the normal level.

\item Paging

This is where the user is reading or skim-reading the document and wants to keep the part they are examining on the centre of the screen as they move slowly and linearly through the document.

\item Section Jump

This is where the user wishes to navigate to a section heading. For example ``I want to go to the `Related Work' heading'' or ``I want to go to Chapter 4''.

\item Visual Browsing

This is where the user is unfamiliar with the document and wants to get a feel for its contents. The user wants to get an overview of the type of information contained in the document, important sections and graphics. This is akin to riffling the pages of a book in order to see approximately what it contains.

\item Visual Search

This is where the user has a known visual target in mind. For example ``I want to find the red graph'' or ``I want to find a particular page and I'll know it when I see it''.
\end{itemize}

\section{The Place of Current Methods within this Classification}

It is clear that the current tools achieve some of these tasks effectively. For example, a scrollbar seems by far the most effective tool for Spatial-Locate tasks. The user can grab the scroll-thumb and move it to the appropriate location. Even if the document is too large for use of the scroll-thumb to accurately acquire the target, it will still be best for the scrollbar to adjust the position as close to the target as possible and then to acquire the target by using Visual Search. End point navigation is also a solved-problem: It is standard practise for Ctrl-Home and Ctrl-End to navigate instantaneously to the beginning and end of a document. Pagination also is easily achieved by use of the Pg-Up and Pg-Down buttons, and most applications achieve Search-For navigation by providing a ``Find'' function or similar search utility.

There seems to be little specific support for Page-Locate and Reacquire actions in most existing systems. Though the Adobe Acrobat PDF viewer displays the number of the current page beside the scroll-thumb when the users users it to scroll, which allows for rapid Page-Locate actions. Past research suggests that position control devices such as scrollbars are actually quite effective at performing reacquire actions \cite{bbkf:1}. This is presumably because a position control interface turns a Reacquire task into a relatively simple Spatial-Locate task.

In our experience, zooming is usually performed fairly rarely within document browsers. Users tend to set the application to a readable level and from thereafter remain in that mode. Nevertheless, many document browsing applications provide the ability for the user to alter the magnification as part of their standard functionality. This functionality is usually implemented from a drop-down list of different settings, or from a tool bar button changing the system mode so that the pointer is a magnifying glass and clicking results in the document magnification changing.

It is in Paging that rate-based scrolling finds its niche.  Paging can be accommodated in small documents by dragging the scroll thumb slowly, however in large documents this is impractical as the smallest movement of the thumb causes the document to jump rather than move smoothly. Use of the Pg- keys is inadequate for the same reason --- the sudden jump these cause disorientates the user who has to re-orientate themselves before continuing to read. Far more satisfactory for this task is rate-based scrolling where the document scrolls continuously at a speed controlled by the user. This allows the user to keep their focus at the centre of the screen while the document flows continuously and smoothly at a rate they control. Panning has often been used to accomplish paging, however rate-based scrolling seems preferable to panning as it is smoother and does not require continuous mouse movement.

Section Jump is performed most effectively by the use of an overview+detail system, where one window contains a listing of the main sections in the document allowing one click to jump straight to them, while the other contains the document itself. A major disadvantage of such systems is that the extra window adds to the screen clutter.

\section{The Place of SDAZ within this Classification}
\label{placeofsdaz}
Current methods do not so effectively achieve Visual Search, and Visual Browsing. It is in these areas (and to a lesser extent Paging and Section Jump), that we believe SDAZ has something worthwhile to offer.

The most obvious advantage of SDAZ is in Visual Search. This was the task type tested in the evaluation by \citename{cs:1} and we believe it was the reason they obtained such positive results. SDAZ provides a unparallelled way of smoothly navigating through large documents while significantly reducing the blurring limitation of standard rate-based scrolling. The rate-based foundation of SDAZ means that scrolling is smooth, while the automatic zoom-out means that the user can navigate more rapidly through the document.

Visual Browsing is facilitated by SDAZ as a result of the zoom-out. Whenever the user scrolls rapidly, the automatic zoom-out allows the user to view a wider area of the document giving them a better feel for its overall contents. In Paging, SDAZ is equivalent to rate-based scrolling since at low speeds there is no difference between the systems. In performing a Section Jump, SDAZ is perhaps not as good as an overview+detail system. However, since section headings are likely to be large and bold, they will stand out when zoomed-out (though perhaps they will not be readable) and could be easily navigated to if the user had a moderate knowledge of where in the document to find the section they were looking for. Also, an overview+detail system usually requires the user to change their point of focus to a different window in order to selection the section, whereas in SDAZ systems their focus is maintained on the document.

SDAZ cannot be properly said to support ``Zooming'' as we have classified it. Certainly the automatic zooming functionality provided by SDAZ is a form of zooming. However zooming in SDAZ occurs to aid visual search, while normal ``Zooming'' as we have classified it here is performed either for obtaining an overview of the document or for viewing an image close up.

In the evaluation of SDAZ performed by \citename{ih:1}, the zoom-out was semantic based --- section headings and graphics were kept fairly large while body text was shrunken further. By using our classification it can be seen that this would have resulted in a trade-off. Visual Search and Visual Browsing capabilities were being sacrificed in favour of Section Jump. Their system did not preserve page layouts which we believe would have reduced its usefulness for performing Visual Search and Visual Browsing, and their system became increasingly an overview+detail system as it zoomed out --- probably increasing its ability to perform a Section Jump.

\section{Using SDAZ with Complementary Methods}
\label{notareplacement}
Having examined the different types of document navigation and analysed the suitability of the current techniques and SDAZ at performing these, we are in a position to judge how SDAZ should be combined with the standard methods.

We have seen that SDAZ should be viewed as an addition to the current arsenal of scrolling techniques rather than a replacement for them. SDAZ represents a modification of rate-based scrolling which is equivalent to it at low speeds, meaning SDAZ performs well in certain types of document navigation which are not handled well by the more traditional scrolling techniques. SDAZ hence should be combined in a system with the standard navigation methods in order to complement them, rather than replacing them.
