Spatial Technologies-Integrating CAD and GIS

Be more accurate and thorough by blending these two worlds.

Everyone has a holy grail—something so precious and important that it seems almost unobtainable. These days my search seems to be for a quick, simple, accurate way to integrate CAD data into GIS applications. The fundamental idea behind CAD and GIS integration is to add a greater level of detail to geospatial data so that geoprocessing and analysis help you be more accurate and thorough when making decisions.

Figure 1. Combining a CAD file with a color relief map generated in GIS helps make landforms more readable.

Fundamental Differences

There has always been a fundamental difference between CAD and GIS. I began using CAD more years ago than I care to admit, with the first beta release of AutoCAD 1.0. About the same time, I started using GIS on an Apple IIe. With AutoCAD, I could draw accurate plans on this really cool monochrome monitor with a green screen, and generate a simple GIS map in a couple of hours. Fortunately, both CAD and GIS have come a long way in the last 25 years. As the two applications have progressed, many users began to look for ways to bridge the gap between the two worlds.

CAD has always been a design tool, while GIS has been used primarily as a cartography and spatial analysis tool. Architects and engineers use CAD to develop drawings for building and manufacturing. CAD provides the accuracy and level of detail to ensure that property lines are laid out accurately, utilities and roads are in the right place and the corners of houses are square.

GIS is used for applications such as mapping, resource management, suitability analysis, routing and network analysis. The major advantage of GIS over computer mapping systems is the ability to analyze spatial relationships.

In a CAD system, spatial data is displayed graphically. CAD files are organized around the concept of layers, with each layer used to help organize graphic information. For example, a typical CAD layer may include topography, utility lines and exterior building walls. Each layer includes different element types because it is viewed as a graphic symbol. On the other hand, in GIS layers are used to organize geographic themes. A GIS feature class layer includes only one type of feature and one type of geometry. This simplifies the process of managing data.

Figure 2. For a recent project in Alabama, we combined a detailed survey with digital ortho quad images to develop base maps for a planning project. By importing the topo data into GIS, we can conduct surface analysis such as as aspect, slope, runoff and viewshed analysis.

Another difference is that CAD is generally used on a project-by-project basis, while GIS is geared toward a longer period of time. CAD has started to address issues associated with lifecycle maintenance, but that is a fairly recent occurrence. GIS is used not only to analyze the way things are now, but also to predict what will happen in the future.

Most architecture firms use GIS either sparingly or not at all. That is not true in the engineering world, where GIS has been integrated into day-to-day practice. Civil engineering firms have embraced GIS more so than others in the engineering profession. In our office, there is a clear distinction between GIS users and CAD users. CAD is still used primarily as a design tool to develop base maps, site designs and construction drawings. Many of the CAD users don't see the need to incorporate their drawings into a larger geographic content. GIS, on the other hand, is the tool of choice for planners.

One reason we are seeing a greater demand for CAD/GIS integration is that CAD users realize there is a benefit to being able to access geospatial data and to define the context for a building site. GIS users demand an increased level of accuracy and detail, and CAD files tend to be very precise because they are generated via traditional survey techniques.

Difficulties in Integration

A recent project I'm involved with in South Carolina illustrates the difficulties facing designers and planners who use both CAD and GIS. We were conducting a two-day workshop with clients to develop a master plan for a 3,600-acre lakeside resort community. Our objective was to select a preferred alternative site, so we had to find a way to quickly and efficiently make good design decisions. To develop a better understanding of the context for the site, we collected geospatial information such as surrounding land uses, vegetation, pedestrian and vehicular circulation, views, drainage patterns, proximity to populated areas and soil characteristics, and organized it all in ESRI's ArcGIS.

Figure 3. Most GIS programs provide some type of 3D visualization capabilities that make it easier to understand the subtleties of a site.

The 3,600-acre site was surveyed, and the resulting data was put in a CAD format for use in AutoCAD. We typically use AutoCAD for all site design work because it gives us the precision needed to develop the final construction drawings used to build the final design.

It was frustrating trying to get the CAD and GIS information to mesh. The GIS data was in several different coordinate systems, and there were discrepancies in the data accuracy. The CAD data was not referenced to a specific point on the earth's surface, but used a Cartesian coordinate system with the local origin set to 0,0,0. The CAD data had also been rotated to fit a standard architectural sheet size, so north was no longer up.

Incorporating the CAD data into the GIS base required that the survey data be distorted to fit the GIS data. This was problematic, because it meant we were losing the accuracy of the site survey, and this would make it difficult to incorporate any analysis back into the original CAD file. Integrating the GIS data into a CAD file required that a lot of information be translated and scaled, and our site would no longer be referenced to the real world. This meant any geospatial information needed for additional analysis would also have to go through the same translation process.

For future projects where site surveys are conducted, we plan to require that the surveys be tied into a projected coordinate system such as State Plane. This will greatly simplify the process of integrating the CAD data into GIS.

Integration Approaches

There are a number of different approaches for integrating CAD and GIS data. Some software developers focus on adding more drawing and editing capabilities to GIS programs, while others seek to add geospatial analysis tools to CAD programs. To make this work, most programs take one of four approaches.

Translate Data. Translation converts CAD data to a GIS data format, and vice versa. For example, a DXF file or a MicroStation DGN file is translated into a shape file. ArcGIS 9 has a bidirectional CAD-GIS translator that can be used to move data from CAD to GIS and back again. Translation works by mapping data from one format to another. Unfortunately, the process is not flawless, and data is often lost. For example, one common problem is that a precise, three-point arc drawn in CAD is typically converted to a series of straight-line segments in a GIS program.

Add CAD to GIS. The simplest form of CAD and GIS integration is to add CAD data as a layer in a GIS application. Most GIS programs can read popular CAD formats such as DGN, DWG and DXF. Within ArcGIS' geoprocessing tool sets there are many tools to manage, modify and move both CAD and GIS data. One limitation of adding CAD as a simple visual layer is that it lacks attribute data, defeating the purpose of putting it into GIS.

Share the same database. Another approach is to have both a CAD file and a GIS file share access to the same database. CAD programs are traditionally file-based, and their development environment has been proprietary and closed in nature. GIS, on the other hand, is based in large part on the ability to share data. Open standards are used by most, if not all, GIS developers, and their software can basically read the same data formats. This philosophy has resulted in the widespread use of GIS, while CAD use has been limited more to the design and engineering professions. Fortunately, this is changing. With CAD vendors increasingly adopting to industry DBMS (database management system) standards, it will be easier to integrate CAD data into GIS applications without compromising the quality and intelligence of the data. With open standards, CAD and GIS programs may soon have similar tools and functionality, so users will be able to go beyond simply exchanging data and focus more on process. Just imagine being able to create a comprehensive slope analysis of a 3D landform and then port all of that into a CAD or 3D visualization program where you can develop detailed site designs.

Use one program. Many CAD systems incorporate GIS tools such as thematic mapping and geoprocessing, and GIS programs are adding simple drawing and editing tools typically used in CAD applications.

ESRI's ArcInfo v8 was a big step in the right direction. It provides better drawing and editing capabilities, including snaps, true curves and other CAD-like features. ArcGIS 9.1 and 9.2 have CAD editing tools such as fillet, inverse and proportion, but they still lack the functionality of the tools in traditional CAD programs. Don't expect to see GIS programs match the quality and precision of CAD drawing and editing tools anytime soon.

Integration Efforts

Efforts to link CAD and GIS within programs date back to the mid-1980s. Bentley introduced MicroStation Geographics, a CAD-based GIS program, more than ten years ago. In the early 1990s, ESRI introduced ArcCAD to help integrate CAD and GIS. In 1988, Intergraph launched its Modular GIS Environment, a complete GIS that uses MicroStation as its graphics engine. Intergraph decided not to continue to enhance the program for the latest version of MicroStation. Instead it's focusing its efforts on the GeoMedia product line.

Bentley's MicroStation GeoGraphics is an extension of MicroStation and the Geoengineering configuration of MicroStation/J. It's a fully integrated CAD and GIS product with both raster and vector functionality. GeoGraphics also includes tools for map projections and transformations. One of Bentley's current efforts is to integrate engineering and GIS at the enterprise level.

Haestad Methods' GISConnect, now owned by Bentley, is essentially an AutoCAD add-on that allows access to ESRI geodatabases and shape files. GISConnect uses ArcObjects technology to access GIS data and ObjectARX to access CAD data. GISConnect basically lets users access ArcGIS tools from AutoCAD.

CADGeoWeave is a subset of Hitachi's GIS Anywhere server-based CAD-GIS connector. It enables AutoCAD and Map users to to access and edit GIS data stored in ESRI, Intergraph, MapInfo, SmallWorld and Oracle products.

Graphisoft's ArchiCAD is an architectural CAD program based on the idea of an intelligent BIM (building information model). The use of APIs helps establish a direct link between ArchiCAD and ESRI products, so the exchange of GIS information and CAD information between the two systems is fairly straightforward.

The ArcSDE CAD Client is a no-cost extension that comes with ArcSDE. It allows some popular CAD programs to serve as spatial data clients to ArcSDE so they can edit GIS features.

Autodesk Map 3D integrates CAD and GIS. Map 3D provides a number of powerful drawing and editing tools that help provide CAD and GIS capabilities. Autodesk's GIS data transformer extensions are add-ons used to enable AutoCAD Map to read GIS data. They include an improved GIS interface that I prefer over other GIS programs I have used. According to Autodesk, AutoCAD Map is fundamentally a data creator and is intended to be used with programs such as MapGuide, which is a data publisher.

At EDAW, we are starting to replace AutoCAD with AutoCAD Map as the primary drawing and editing tool, making it easier to integrate CAD and GIS on all of the projects in the office. Data from AutoCAD Map is also designed to plug into MapGuide, so it will be easier for us to publish geospatial information on the Internet. One complaint I have heard from CAD users is that Autodesk Map is not as responsive as AutoCAD. As a result, at least one EDAW office has shelved plans to switch over to Autodesk Map for the time being.

ESRI has launched an initiative to enhance CAD-GIS integration. Included in this initiative are efforts to create intelligent CAD translators, enhance CAD-like editing in ArcGIS, support a wider range of CAD formats, add support for 3D CAD objects and create new engineering applications with business partners.

Building Block

Some industry experts predict that CAD will return to its roots and be used exclusively as a design and engineering tool, while GIS continues to expand and become the building block from which all planning decisions are made. I believe that the future of GIS depends on its ability to incorporate CAD data, because such data provides a level of detail that the world of GIS is seeking.

James L. Sipes is the founding principal of Sand County Studios in Seattle, Washington, and senior associate with EDAW in Atlanta, Georgia. Reach him at jsipes@sandcountystudios.com.