man-made features on a coordinate reference system. Different line, symbol, and area colors are used to aid in distinguishing between water features. man-made objects, wooded areas, and contours, A line map is produced from scribed, inked, or pasted-on line copy. A photographic map is a photograph or assembly of photographs on which descriptive cartographic data, marginal information, and a coordinate reterence system have been overprinted. The photographs may be uncontrolled, nominally vertical aerial photographs, or they may be rectified photographs, with image displacements due to camera tilt removed. An orthophotographic map is similar to a photographic map with the exception that, in generating the orthophotographs from conventional aerial photographs, image displacements caused by both camera tilt and terrain relief are removed. Photographic images on an orthophotographic map are therefore in their correct orthographic map position. Digital maps have evolved in recent years with the development of powerful data-processing systems that have made it possible to collect and store digitized map data. Manipulation and merging of the digitized data and selective retrieval of desired levels of map information, either in graphic form as a plot or a printout or in numerical form as a body of data, make the digitized representation of map information (virtual map) a very flexible form (Thompson, 1979).
Each of the forms of map information (line map, photographic map, and digital map) has its advantages and disadvantages as candidates for a base-mapping medium in a multipurpose cadastre. The majority of map-producing organizations today are producing line maps. To a considerable extent, a line map can selectively control the type and amount of information to be shown on the base map. However, line maps are the most difficult and expensive to update in a timely fashion. Photographic maps can be readily updated with the collection and processing of new photography, and they contain a large amount of terrain surface detail. As a base map, however, the photographic map may have more detail than desired, without the possibility for control of the type and amount of information to be shown. Image displacements in the photographic map due to camera tilt and terrain relief are removed in the orthophotographic map, with the additional expense of the differential rectification process necessary to produce the orthophotograph. With the development of the techniques of automated cartography, digital mapping promises to be the form most responsive to the requirements for flexible selection of type and amount of base-map information and for regular base-map updating. Only in this form can map information that has been collected at different scales and in different formats be efficiently merged, digitally, and displayed together. Necessary digital mapping standards will evolve as this new technology matures in future production mapping environments. Their development is being advanced currently by a Committee on Digital Cartographic Data Standards organized in 1982 by the American Congress on Surveying and Mapping, with the sponsorship of the U.S. Geological Survey (Mocllering, 1982).