Many of the mechanisms employed in the RP interface are not well supported in the (currently available) standard WWW/HTTP/HTML environment. In particular, the ADL WP must operate within the following WWW limitations: 1) current implementations of HTML lack mechanisms for presenting vector data, and provide weak support for entering spatially-indexed information; 2) HTTP is stateless, and is designed for small, fast transactions; 3) current Web browsers do not allow a sufficiently high degree of interactivity. Helper applications are only a short-term substitute for better browser-helper communications and/or programmable browsers.
The limitations of HTML are apparent when attempting to support a user in defining a spatial search region. A natural procedure for such a specification is to draw a polygon on a base map by either clicking on multiple points or clicking and dragging over a desired region. Such actions are not currently supported by Web browsers, which immediately send an HTTP request after a single mouse click.
The stateless nature of HTTP makes browsing difficult. By default, after a user completes an HTTP request from a WWW client to an HTTP server, neither the client nor the server maintains any state or ``memory'' of the transaction (other than perhaps logging the URL involved). Each request appears to the server to be completely new. This statelessness prevents needed activities such as a user-defined configuration, environment parameters, a query history, and iteratively refined searches. In order to support these activities, information must be kept at either the client site (through parameters stored in the URL and ``hidden'' form variables) or at the server site (through unique user identifiers and a session database).
Finally, no WWW browser that we know of supports vector data display. This is a serious issue, since a significant and important portion of spatially-indexed information collections involve items represented in vector format.
In addition to the three major UI functionalities supported in the RP, the UI of the WP is designed to be user-configurable, and to permit delivering data to a remote user. Most WWW information systems have a simple static UI and a static representation for retrieved information. A static UI is insufficient for systems that must provide a wide range of services to heterogeneous library user communities. To provide various services to different levels of users, a UI should be user-customizable and provide a mechanism for saving tailorable options for future sessions. Additionally a user must be able to retrieve a particular data item or metadata record. Since the WWW is part of the Internet, simple retrieval via FTP is straightforward to implement. As noted above, however, the items in ADL are often extremely large, so methods that allow for the extraction and transfer of smaller pieces of a data item (portions of images or lower resolution versions for initial browsing) are needed. Such a requirement is in part supported by the use of wavelet transformations, as described below.