Hypertext navigation is also restricted by the physical limitations of the display screen. The inability to view large amounts of information at any one time due to the small size of computer displays falls under the category of context-in-the-small problems [Nielsen,1990a]. This includes the issue of readability, when the contents of a node do not fit into one screen and have to be carried over to other screens. Larger displays can only partially solve this problem. Conventional scrolling techniques use arrow keys or paging keys allowing only vertical movement to various places on the screen. While scroll bars do allow two-dimensional navigation, it is not easy to focus on a particular region of interest.
In an effort to improve the display of large two-dimensional spaces, researchers at the University of North Carolina, Chapel Hill, developed two similar direct manipulation techniques. Both use a miniature of the entire information space to assist the user in remembering his or her location. The first technique allows the user to rapidly "roam" over the space while the second allows "zooming" into a particular region while roaming [Beard & Walker, 1987].
The entire information space can be shown, in miniature form, in a map window occupying a small part of the display. A wire-frame box or rectangle inside the window shows the portion of the information space displayed on the main display. The main display is the actual viewport into the information space. Thus, the map window provides a clear sense of location within the information space. the size of the wire-frame can be changed using the mouse thereby zooming in and out of the region. Also, the wire-frame box can be dragged around the map window thus roaming around the information space. Experiments proved that the roam and zoom features were significantly faster than vertical and horizontal scroll bars thus improving performance.