The use of GIS has been extended to include the third dimension where users decide that their spatial datasets can be viewed most realistically if placed in a real-world setting. One of the common 3D models is a comprehensive land-related information system which includes land use, buildings, flood hazard, income levels, population at parcel level, and such like.

CAD Shortcomings
Traditionally CAD files are found in organisations such as utilities and mapping agencies, which typically focus on display and presentation of maps rather than on analysing data. The introduction of GIS has extended the responsibilities of such organisations by making available their large pool of datasets to assist in decision-making processes. One organisation that plans to benefit from such innovative use of its data is Troy University. To generate the 3D model discussed here, use was made of a digitising table connected to AutoCAD and an existing Computer Aided Drawing (CAD) file manually created in April 2002 by the university’s maintenance department from a topographic map scale 1in=50ft with 1ft contour interval. The map projection was UTM Zone 16. Generating a 3D model was not planned; the CAD file was created to assist the maintenance department in their daily activities. As a result, the data structure of the CAD file was intended only to capture and display a digital map. For direct application within a GIS environment the CAD file showed the following impediments:

• contour lines broken when passing through buildings and where text was placed
  
• road edges not continuous; lines broken when passing beneath tree cover
  
• cluttered map; features and text made map difficult to read
  
• layer names not intuitive; features difficult to identify
  
• features digitised; as was all content of topographic maps, including grid lines.