The principal point has long been regarded as one of the fundamental parameters in camera calibration. In the age of film based aerial and large format terrestrial cameras, the principal point could be located by a variety of techniques with a certainty of ϩ Ϫ 10 m (Carman and Brown, 1961) and this was considered sufficient. However, aerial cameras were precision, purpose built, expensive pieces of equipment where the assembly was painstaking and the location of the principal point measured to a known tolerance. In the digital era, photogrammetrists, and many others, are using cameras which have not been specifically designed or built for photogrammetry. For these cameras there is no requirement for the manufacturers to position the lens in a pre-defined location relative to the image sensing plane or for the lens manufacturer to align the lens elements precisely. In fact, deviations from the centre of the sensor can be a considerable percentage of the extent of the sensor (up to 10 per cent for some zoom lenses (Burner, 1995)). This paper discusses the development of methods of obtaining the location of the principal point, considers the relationship between the principal point and other parameters in the functional model, and shows how the location of this point can be estimated with and without recourse to autocollimation methods.

This paper reviews some of the past Okapi research and discusses the role of Okapi in the current TIPS project. The main purpose is to report new challenges faced by probabilistic text retrieval in the web environment and to indicate some of the solutions that are currently under investigation. In this context, extraction of indexing units from formatted document sources, user interface design, implementation of field searching and query expansion within the framework of probabilistic searching are discussed. The problem of maintaining session continuity in the web environment and a possible solution to this problem are outlined. Other challenges posed by the open nature of the web environment are also indicated. These include the difficulty of delimiting the boundaries of a search session and the potential of the web for collaborative information retrieval. A system for collaboratively filtering documents based on their contents is described in this connection. Issues surrounding the integration of Okapi with other pieces of software being developed for the TIPS project are also briefly discussed. 50 1 2