Unlike in a GIS, all data represented by a CAD dataset is normally contained in a single source file. This includes geometry as well as nongraphic information such as feature attributes and graphic properties that define symbology.
This topic explains what comprises CAD geometry and how DGN and DWG formats organize data.
CAD geometry is not intrinsically a GIS feature class. When you connect to a CAD file in ArcGIS Desktop, the geometry is translated on the fly into virtual feature classes that resemble geodatabase schema.
MicroStation and AutoCAD platforms also have the ability to link geometry to external database records similar to a GIS, but this is less common and tends to be utilized more with vertical solutions or on-premises workflows.
2D CAD data consists of geometric primitives. It is created on a Cartesian plane at a constant z-coordinate. The z-value in a 2D context is referred to as its elevation and is not necessarily zero.
Examples of 2D geometry include the following:
- Line segments and closed polylines representing polygons
- 2D planar regions constructed of three or more edges
- Arcs, splines, and circles
3D data can be any data that spans multiple z-elevations—including 2D features. However, 3D CAD objects that represent discrete three-dimensional objects fall into two basic categories: 3D solids and 3D surfaces.
- 3D solids define volume. They range from simple constructs, such as a box, to detailed virtual models that represent real-world objects. 3D solids can be used to obtain mass properties information such as weight, center of gravity, or inertia values.
- 3D surfaces define boundaries or shells. They can be used to model objects or data points that produce complex free-form curves. Surfaces created with nonuniform rational basis splines (NURBS) generate mathematically precise surfaces that are true curves. Surfaces constructed of three- and four-sided faces generate polygonal meshes that approximate curves.
CAD layers organize data similar to layers in ArcMap; however, they do not follow the GIS simple features model. CAD authors are free to mix geometry types and other data on a single layer. It is also possible to use line type and color to further classify the data. As a result, the data's context, along with textual information and a significant amount of human interpretation, can be required to identify geometry as a particular feature in ArcMap.
Annotation in a CAD file is measured in map units when it is created. Typically, single-line and multiline text are stand-alone graphic elements that have no inherent ability to link to geometry. They are organized into styles that can specify a font, color, and size. Font formats vary and can include a mix of proprietary, vector-based, and TrueType fonts.
CAD data can also include page layout elements such as title blocks and legends. However, this is typical of older legacy CAD files and less common with newer datasets. Both AutoCAD (Release 11 and higher) and MicroStation V8 utilize separate name spaces that are similar in purpose to the ArcMap layout view. Similarly, they allow the CAD author to store page layout elements away from the actual data in the same file.
In AutoCAD, page layouts are created in paper space, while in MicroStation they are created as sheet models.