Computer aided design systems as their name implies provide a means of designing objects. For the architect it will be some form of building design, for the electrical engineer it may involve a complex wiring diagram, for a civil engineer it could be a highway, tunnel or bridge project and for a mechanical engineer it could be some form of engine or pump design.
GIS is often about querying information and using various tools to show the results within the medium of maps. A graph can take data and give a visual representation of the information often in more meaningful form than a spreadsheet. However sometimes this data can be more clearly understood through the medium a map. For example often thematic maps can give a more meaningful understanding of complex data than a spreadsheet or a graph of the same data could.
CAD enables engineers and architects to be creative in an incredibly more accurate way than was ever achievable through a drawing board. Whilst everything has to be at a scale on a drawing board, because it is an approximation of the object being designed, in CAD the object is created in the software at full size. That can be the design of a huge building or a tiny instrument. In both cases CAD software allows the designer to produce the object without having any concern to scale. Only when the object is being output to a design sheet is scale involved. Dimensions are often automatically added at this stage and in some cases software can also create a bill of materials.
Working within a GIS system is often working out relationships between objects. For instance the distance object a is from object b, or whether object a is within object b. To achieve this sort of outcome GIS relies on SQL to create the appropriate query statements of a geodatabase to find, in many instances, topological related answers to the problems GIS specialists need to answer.
CAD is very good at creating accurate 2D and 3D models which can dimensioned and prototyped prior to creating production models. However a CAD system is not designed to manipulate maps or the attribute data associated with them.
This does not mean to say that there is no overlap between GIS and CAD. As previously mentioned both systems are computer software solutions which use layers to convey information concerning the objects being designed. With maps the GIS is used to create point, lines and polygons to represent all types of geographical objects such as towns, roads and states which are placed on layers within the software. CAD uses layers to make it easy to design with dimensions on one layer for example and other layers breaking down various elements of the design. The completed design is then scaled to fit on the printed sheet for distribution.
AutoDesk has developed mapping software to complement its 2D and 3D CAD software. Linking CAD drawings into a GIS system is quite common where there is an overlap between the two software solutions. GIS is not really appropriate for designing within an architectural or engineering environment and CAD is not the most effective tool for creating maps. The overlap however will occur where there is a need to combine the two solutions when appropriate.
A university for instance may have a requirement to upgrade a facilities management system which incorporates both CAD generated building plans and GIS generated maps of the campus layout. In this scenario there is a requirement to have some form of integration between to the two systems. The campus maps give a location based aspect to the problem and the building outline plans have to be integrated to enable an accurate calculation of the various departments square footage occupation of university buildings and therefore what their financial contributions to the university should be. If the maps of the campus are currently in a paper format they would need to be scanned and then the raster output would have to be projected onto the campus map to give an accurate representation within the GIS system. Alternatively the raster data may be vectorized in order to use the maps to calculate areas and perimeters of the various campus buildings. GIS is good at this kind of work and can then be integrated with a relational database system to form the basis for facility management system which could generate reports. This GIS based solution could include an automated invoicing system for the faculty departments which would be based on the square footage they occupy.
Of course to have an acceptable solution a GIS consultant may have to link all of the components: GIS, CAD and relational database integration in an acceptable format for the end user. This could be achieved with the help of customisation. GIS systems such as ArcGIS, MapInfo and QGIS can be customised with programming languages. ArcGIS and QGIS can be customised with Python and MapInfo has MapBasic to achieve the same aim. AutoCad has been very successful primarily because of its ability to be customised by using the AutoLisp programming language. Again both GIS and CAD are similar in that they both rely on programming languages to enable customisation.
One of the most obvious similarities between GIS and CAD is that they both use layers within the software in the creation of models within the software. However these two software solutions address different computer based problems with GIS stronger when dealing with topological and information based problems whilst CAD is the best solution when design is the primary objective.