Pima County Department of Transportation

Geographic Information Services Division

What's the Relationship Between CAD and GIS?

At the 1998 Autodesk Design World, Bill Wittreich of Wittreich & Associates, gave a presentation titled Beyond CAD into GIS. What follows is paraphrased from a portion of his talk.

AutoCAD Map adds GIS features to the base AutoCAD product and, as such, these features help define the difference between CAD and GIS. In short, the difference between CAD and GIS is the difference between a drawing and a spatial database.

Modelling

CAD models things in the real world. GIS models the world itself. Therefore, GIS uses geographic coordinates systems and world map projections while CAD coordinates are relative to the object being modeled and are not usually relative to any particular place on earth.

Objects

CAD objects include lines, circles, arcs, text, etc. using layers, blocks, internal data, and dimensions. CAD objects don't know about each other, even though they may touch or overlap.

GIS objects know about each other:

• GIS understands networks. For instance, the lines describing streets are related to one another.
• GIS understands enclosed areas (polygons) and their associativity with other objects.
• GIS understands connectivity, conductivity, and associativity which enables spatial analysis.

GIS adds topology

The primary difference between CAD and GIS is topology. GIS has it, CAD doesn't. In a CAD environment, the objects (lines, polylines, points, etc.) have no relationships between them. Topology brings these objects together into logical groups to form real world models.

• Node topology allows spatial analysis, such as buffering to determine other objects within a certain range.

• Network topology allows modeling of direction and resistance. Path tracing finds the fastest or best route. Flood tracing determines the maximum flow from a given point and network resistance. As with node topology, buffer analysis can be applied to networks too.

• Polygon topology enables polygons to have relationships. Polygons also have centroids which can be used to hold data relevant to the polygons. Polygon spatial analysis includes overlay analysis such as determining parcels in a floodplain. Polygons can be "dissolved" using attributes with common values to remove interior lines, in effect aggregating polygons with in the same class.

Topology and spatial analysis differentiate GIS from CAD.

Data Management

GIS separates object storage from object display, combining data form multiple sources into a virtual data warehouse. That data can then be used in any number of separately defined analyses or presentations. CAD systems carry baggage such as line color, line width, etc. that is not relevant to the data itself.

GIS systems are usually disk-based and can model larger areas than CAD implementations which are usually memory-based. For instance, CAD files are typically smaller, such as product designs as compared to regional, state, or even world models in GIS.

The Trend

While the distinction between CAD and GIS is grey now, as features are added to CAD systems, the distinction will blur even more.