What is CAD data?

Computer-aided design (CAD) drawing files are digital representations of man-made designs or real-world objects. They often serve as legal documents or proposed plans and are used primarily by the technical professions that include engineering, architecture, surveying, and construction disciplines.

ArcGIS supports the following CAD formats:

Software title	File format
AutoCAD	.dwg and .dxf
MicroStation	.dgn

Both AutoCAD and MicroStation CAD drawing files are stored in file-based vector formats. Generally, all data comprising the contents is contained in the source file. This includes any non-graphic information that may exist as property values or feature attribute data. Both platforms also support extended functionality to link objects to external database records; however, this is less commonly used.

Geometry

CAD geometry includes a range of mathematical descriptions. In addition to points, lines, polygons, and annotation, CAD data often includes additional representations such as 2D or 3D arcs and splines and 3D planar or free-form surfaces. ArcGIS will translate and group supported elements into homogeneous GIS feature types using semantic translation techniques specific to AutoCAD and MicroStation platforms.

It is common to find title block and other page layout elements included with CAD feature data. This is less frequent with more recently created DWG or DXF files because AutoCAD supports two distinct workspace environments similar to ArcGIS layout and data views. ArcGIS will only read information from a DWG data view referred to as modelspace.

Annotation

CAD annotation uses proprietary or vector-based fonts as well as TrueType fonts. Properties such as font type, size, and color are defined and grouped using text styles. CAD text styles and their properties are exposed in ArcMap through the layer properties dialog box Fonts tab.

AutoCAD and MicroStation create text as stand-alone entities independent of feature geometry. With the notable exception of AutoCAD block attribution, CAD annotation will translate with no ability to link to feature geometry or directly support ArcGIS labeling. ArcGIS provides several tools to perform spatial joins and conversion of CAD annotation to a geodatabase.

Units and coordinate systems

AutoCAD and MicroStation CAD files are based on the Cartesian coordinate system. This system assumes an orthogonal world without distortion and without a reference to a geographic datum. This enables the creation of extremely precise geometry that can be positioned anywhere in 2D or 3D space. Likewise, units of measure are not bound by the CAD file's coordinate system. As a result, scaling and other transformations may be required when using CAD data in ArcGIS.

Layers and styles

Unlike GIS, CAD layers may contain multiple element types that represent a single geographic feature. In addition, layer, line type, annotation, and properties such as color, and styles may also be used to further classify CAD features. Many of these properties are exposed in ArcGIS property dialog boxes and can be used for building highly granular selections and definition queries to isolate specific CAD geometry.