One of the fundamental changes to cartography during the 20th century has been the introduction and use of computers and computer-driven machinery (especially drawing and graphic devices) to mapping. There are many ways to encode graphic images in digital form such as with Cartesian co-ordinate geometry, and computers are efficient tools for encoding, storing, analysing and displaying graphics.
Numerical data representing map images or geographic phenomena may be collected from aerial photographs or satellite images, or by tracing from existing map documents. There are numerous ways that such data may be collected. Resembling a drafting table with a pointer attached by an electronic wire, a cartographic digitizer measures the position co-ordinates of that pointer and records them in a computer. The position of points can be recorded at frequencies high enough to sense every nuance of hand movement. The first patent holder for a cartographic digitizer is A.R. Boyle of the University of Saskatchewan.
Also available are devices which automatically scan documents into data representing a fine-resolution raster grid. They require complex computer programs to convert them into forms usable for mapmaking.
The stored map may be processed by the computer through algebraic computations between recording and plotting. This opens many possibilities, such as bringing together data representing overlapping or abutting maps, or data from different types of maps. Arithmetic operations required for changing scale or transforming the map through the various map projections are also easily done, and maps in the form of numerical data can be transmitted over telecommunication lines. Maps at different scales and of high graphic quality, with elaborate and accurately drawn symbolism, may be produced from the data in amazingly short time periods.
A major problem with numerical map data is finding and correcting errors. This can be mitigated by displaying and manipulating the information on screen. Features identified by pointing to the graphic image may be moved, stretched, squashed, modified or erased. While the operator manipulates the image on the screen, the numerical data from which the image is generated are simultaneously altered. Systems designed for such functions are often referred to as "interactive mapping systems."
Other types of data manipulation are possible, eg, the mapping of the 3-dimensional characteristics of topographic maps. A surface can be numerically recorded and different types of symbolism, such as relief shading, contours or perspective views, can be generated from the data record. Another important application is in overlapping information from maps of different subject matter of the same area. A map of soil capability may thus be compared with a map of current land use, and the areas of each class of match and mismatch may be listed in a convenient table. Systems designed for such calculation are generally termed geographic information systems. Computer mapping has also made possible the mapping of statistical, socioeconomic and demographic data (eg, census information). Such maps were formerly rare because the tedious handwork required to produce a single map was not often seen to be worth the effort.
Computer-assisted mapping has fundamentally changed cartographic design. In traditional mapping, the designer had to decide on the final choice and position of every feature on the map before drawing the first feature. Now anything can be changed in computer-assisted mapping and design rules have lost some of their rigour. Equally, the map is no longer the printed white sheet but has received depth, ie, there is often a hierarchy of information (graphic, textual, etc) that can be retrieved by clicking information icons.
There are many computer-assisted mapping programs funded by the federal and provincial governments in Canada. The electronic atlas project (see National Atlas of Canada), hydrographic charting, statistical mapping systems and many research activities of government agencies and universities have made Canada one of the leaders in the field.