ESRI CAD Product Manager
redlands, CA, usa
Email: [email protected]
Many perceive GIS and CAD to be competing technologies because of a superficial similarity in their respective displays on a computer monitor. Indeed, a certain amount of functional overlap does exist, particularly in the areas of computer graphics and spatial data entry. Clearly the two systems need to work together and information needs to pass back and forth between them
The distinction between Engineering and Geographical Information Systems tasks is beginning to blur significantly. Isolated processes that involve calculations, drafting and research are beginning to come together within the powerful set of graphics, databases and information management tools found in GIS. While CAD has been the traditional cornerstone of engineering solutions, GIS is emerging as a more integrated alternative for engineering design and modeling.
Many perceive GIS and CAD to be competing technologies because of a superficial similarity in their respective displays on a computer monitor. Indeed, a certain amount of functional overlap does exist, particularly in the areas of computer graphics and spatial data entry. Clearly the two systems need to work together and information needs to pass back and forth between them. A GIS frequently uses data encoded into CAD drawings as a source of information describing facilities. Likewise, GIS can provide spatial and geographic context to a CAD drawing.
The basic difference is in how each technology stores data in the computer and the specialized tools that are developed to leverage that data. CAD is rooted in drafting-it is used to create and represent graphic features as drawings in a computer. GIS is rooted in data management-it has the capability to process geographic features and their related attributes in a computer database.
The CAD data model stores geographic data in binary file formats with representations for many varied forms of geometry. Entity attribute information is represented using various loosely linked encoding strategies in CAD files.
GIS is database-centric and, thus, handles data in a single seamless spatial database. Because GIS has always held the possibility of managing data over a wide geographic extent, it includes many tools for map projections and handling large data volumes. GIS employs the concept of layering for segregating different kinds of information into more easily managed layers. In CAD, layers are used to simply organize graphic data into layers of information to facilitate the drafting process. In GIS, layers are used to organize geographic themes, including the graphic definition of features, topology defining the spatial relationships between features, attributes defining the characteristics of the features, and business rules.
Additionally, the speed of GIS for accessing data and generating maps is significant. GIS technology offers data analysis capabilities and a range of functionality that clearly distinguishes it from the drawings produced by a CAD system.
CAD – GIS Interoperability
Many organizations have both systems operating to perform different mission critical tasks. Organizations want to access CAD data to build GIS content and for direct processing. These same organizations also need to provide GIS data in CAD format to give context to new design that may be drafted in CAD. Over the years GIS consulting firms have designed data translation routines that have evolved from cumbersome multiple processes to automated processes. Translation is not a unilateral process. CAD files are inherently different from GIS data files. GIS data elements are typically broken into three distinct categories – lines, polygons, and points. CAD files are often constructed using a variety of graphic primitives that may include several CAD objects to define what would be a single GIS object.
CAD authors will often use whatever techniques most expedient to them to express their design intent as a drawing. Too further complicate matters, CAD files usually have different element types and different features all stored in the same file and even on the same CAD layers. A GIS feature class layer, on the other hand, consists of only one type of feature (parcels) and one geometry type (polygon) at a time. Each element in a GIS file is stored as a spatial database record. Again, this is not the case in CAD files. CAD files are stored with symbolic attributes; level, color, line weight, and style properties associated with each element that can help distinguish one element from another. The usefulness of CAD drawing as GIS content varies significantly depending on the accuracy, adherence to arbitrary symbology standards, and use of any geographic spatial reference. Because of the lack of standardization in CAD systems, data integrity, data consistency and data quality are major hurdles to overcome in GIS and CAD Interoperability.