Computer-aided drafting (CAD) is an automated system designed to efficiently and accurately create and display graphic entities at a high level of precision, primarily for use in architecture, engineering, and mechanical design. It is important to note that the CAD acronym is often expanded in a number of synonymous ways: The A can stand for “aided” or “assisted.” and D for “drafting” or “design.” Other related terms are computer-aided drafting and design (CADD), computer-aided mapping (CAM), and computer-aided cartography (CAC). CAD is often used to generate parcel, street, and utility maps, which can be used alone or with GIS.
While CAD is similar to GIS, there are several important distinctions between CAD and GIS, and by comparing these systems, we can best describe CAD. The primary distinction is that CAD is designed to create and edit graphic entities and generally has minimal database capabilities, while GIS is a spatial database that uses graphics to display the results of analysis, with graphic editing being a secondary capability. It is worth noting that some CAD programs do provide GIS functionality as an add-on to their core graphic editing functions.
In CAD, properties such as layer name, display color, display width, and text can be attached to graphic entities. In some cases, nonspatial attributes can be attached to specialized point entities. However, these data are generally not available in a tabular format within CAD. In GIS, entitles are directly linked to a database that contains geometric information as well as nonspatial attributes and is readily available in a tabular format.
In CAD, all the graphic entities are generally contained within a single “map” file and are available only through that file. A GIS “map” is a collection of pointers to multiple data files that can be used in other GIS maps.
CAD programs can organize graphic entities into “layers,” which are primarily used to control the display of entities by defining colors and linetypes for different groups of objects. CAD layers can also be used to organize entities thematically. For example, all entities that are used to draw roads, which can include points, lines, polygons, hatching patterns and text, could be assigned to a layer called “Roads.” The entities could be more specifically classified by assigning them to drawing layers named “Arterial Roads” or “Local Roads.” In GIS, the primary organizing factor for entities is geometry type—points, lines, polygons— which are then organized into thematic layers, with specific classifications such as “Local Road” being an attribute in the relevant file within the database. In CAD the “layers” are created in the CAD map and exist only in that file, while in GIS, each “layer” is a pointer to a separate data file.
CAD generally does not create topology. If a series of lines is connected to form a polygon, CAD recognizes this as a polygon only under special circumstances and cannot recognize that a point within that polygon is related spatially to that polygon. Lacking this type of topology, spatial analysis is limited in a CAD program.
John Michael Schaeffer