It is quite apparent that engineering and planning professionals rely on digital data to design, map, and analyze infrastructure.  CAD and GIS (geographic information systems) software tools are often used to support these engineering design and geographic analysis activities.  These technologies are distinctive; however, there is increasing demand for CAD and GIS software that is well integrated to help professionals exchange data and collaborate more efficiently throughout a project lifecycle. 

There are many difficulties with CAD and GIS integration-data often lose connectivity, accuracy and geometric precision.  Even with the newer versions of both CAD and GIS software, which are more accommodating to such integration, integration is not without its challenges.

CAD

CAD is used to represent and model real-world objects and to move and edit these objects with no loss of precision.  CAD has become the primary tool for engineering teams to design infrastructure facilities and for surveyors to produce topographic plans and subdivision layouts.

The actual meaning of the lines that are drawn in CAD are inferred from the context of the overall work product or coded into standards of symbology.  CAD drawings are most often file based.  Drawing entities take their attributes (color, linetype, and feature type) from the layer on which they are created.  While this is an effective way to organize data it requires careful quality control to ensure consistency.

Attempts to use CAD for GIS data creation and management are hampered by several limitations, including:

• Lack of data connectivity and topology      
• File-based storage of data
• Single-user access to data and associated information      
• Crude methods for attaching attributes to features
• Difficulty in maintaining data integrity and metadata in a CAD environment      
• Slowness of updating spatial data due to many steps to convert from CAD to GIS
• Loss of accuracy due to data conversion      
• Trouble controlling data standards.

Basically, CAD data can be drawn in such a way that it resembles the data structures and organization of GIS, or it can be a jumble of inconsistent data and loosely enforced standards that just looks the same as a well-organized GIS map when printed.

GIS

GISs are electronic systems used for the storage, retrieval, manipulation, analysis, and display of geographically referenced data.  Most often, GIS uses a series of points and short line segments to represent a geometric arc, whereas a CAD geometric model uses a mathematical formula to generate true arcs.  In GIS, the meaning of data is described in a database context with a predefined geometric representation, optional topological business rules, and a tabular database schema.

Traditional GIS was aimed at general cartography and broad land-use analysis, not precision design for construction. Although GIS can approximate the geometry of designed objects, such as roads and communications networks, it cannot represent them with the geometric precision that many engineers demand.

Early use of GIS for design projects was characterized by several limitations, including:

• Loss of geometric accuracy and precision of features      
• Limited data editing tools
• Limited engineering drawing entry and data migration capabilities.

Integration Needs

Engineering, planning, and GIS/CAD professionals and their clients now require much tighter integration-they need precision data capture, creation, and maintenance tools, whether for surveying, mapping, or engineering design.  These tools must be fully integrated with database and analysis capabilities as well.

These same users also want full process and lifecycle integration so they can pass digital geographic or design data transparently between project team members at any point and later to users, such as operations managers, facility managers, field technicians or the general public.

In summary:

• Users need to be able to create both traditional GIS features (points, lines, and polygons) as well as features with true geometry, including arcs and circles.
• Data passes through a lifecycle or workflow, from surveying and mapping to design to construction to management, and the same data needs to be used in many ways by many people.      
• Data needs to be integrated from many sources, including CAD and GIS environments, spreadsheets, and databases.
• Whether printed or published on the Web, design documents need to be accurately rendered with the symbology and appearance that users expect.  
     

Approaches and Guidelines to Achieving Integration

Newer Software.  Newer integrated software overcomes many of the limitations of traditional products by managing both CAD and GIS features in a seamless and precise database environment.  These solutions create, integrate, and manage CAD and geospatial data with no loss of precision or topology, provide tools for performing geospatial data maintenance, and support relational database management technology.

One of the challenges to overcome, however, is the problem of ensuring data integrity when data are not uniform and coming from multiple locations.  Feature templates can be created that provide default values and limited editing capabilities.  These templates can also automate layer selection and object table attachments.  These templates are portable and can be modified to fit various installation needs.

Standards control is also important in this context and users are often encouraged to use a standards library and to develop a mechanism by which to adhere to standards.

Unlike older, more proprietary systems, these newer packages stress open architecture and being able to make data stored in a single location accessible to multiple software vendors.  Often, accessibility is provided through Web services in either Intranet or Internet configurations. Another key goal of many of these newer systems is to streamline data conversion and eliminate the loss of precision (and time) due to frequent importing and exporting.  Tools are available that allow for the reading/writing of spatial data stored in a central database, such as Oracle Spatial.

Traditional Tools.  A number of guidelines and procedures can be followed using traditional tools to achieve CAD/GIS integration.  The usability of CAD drawings as GIS content can be greatly improved with minimal effort during the CAD drafting process-avoiding methods that can negatively impact the quality of CAD data for use in GIS and following a few strategies for data construction and organization.

When building CAD drawings, it is useful to be able to isolate objects that can be candidates for GIS features.  There is a great deal of flexibility within CAD to organize data.  The key to successful use of CAD data as GIS features is the ability to uniquely and consistently categorize different objects within the CAD file that can be used within GIS.  GIS can directly use the geometry of the CAD objects as the GIS feature geometry once the correct features have been successfully identified in the CAD file.  These methods are recommended even when the data are not intended for GIS use, as the needs may change in the future.

There will be times when the optimum method for constructing a CAD drawing will include objects and techniques that would make it difficult to use in GIS.  By identifying these up front, alternative methods to enhance interoperability may be considered. 

More to Come

There are a number of other guidelines that should be considered and then enforced as part of an organization's CAD standard where appropriate.  Some of these will be discussed in an upcoming article.