Types of user-created attributes in CAD drawings
User-created attributes in a CAD file associate nonspatial information to geometry. DWG/DXF and DGN formats each use different methods for storing user-created attributes.
This topic describes the types of user-created attributes you can encounter when working with these datasets.
Block references and attributes
A DWG block reference is a named collection of graphic elements and user-created attributes that are treated as a single element or object. One or more instances of a block reference can be inserted (or copied) in a drawing. They are commonly used to symbolize point features—similar to marker symbols in ArcMap.
A block attribute is a nested component of a block definition. It is a textual element that is used to store information with each block inserted in the drawing. Block attributes are created with an attribute definition that defines how the attribute is stored and if it is visible or not visible. It has a name and a value—similar to field schema in a database table.
Extended entity data (Xdata)
DWG extended entity data (Xdata) stores user-created information similar to block attributes except that xdata attaches directly to geometry. Using Xdata requires custom programming written in AutoLISP or the .NET languages. It can be attached to geometry programmatically or interactively by the user with a custom dialog box or command prompt interface.
DWG xrecords are similar to Xdata except they are object based and have fewer limitations for software developers. They can store information at the drawing level as well as link information to geometry.
Feature classes stored in DWG files
Feature classes stored in DWG files are named collections of standard DWG geometry and other native CAD objects that share the same properties and the same attribute fields. Their schema and encoding structures are defined by the Esri mapping specification for CAD—a CAD-GIS interoperability specification.
These types of feature classes are stored at the drawing level as empty (schema-only) feature class definitions. They define filtering criteria that ArcGIS for Desktop uses to identify a particular subset of geometry as a feature class. Similarly, attributes attached to geometry with matching schema are also filtered as constituent attributes.
At the encoding level, these feature classes consist of DWG xrecords that are stored in a standardized framework of DWG object dictionaries.
Feature classes stored in DWG files can be created in AutoCAD-based applications using the Esri plug-in ArcGIS for AutoCAD or generated with the Export To CAD tool in ArcGIS 9.3 or higher.
A DGN tag is a textual element used to link information to geometry. It is linked using the geometry's unique ID. A tag set definition defines how the tag is stored and if it is visible or not visible. It has a name and a value—similar to field schema in a database table.
A DGN MSLink is a nongraphic data element that links geometry in a DGN file to an external database table. Other software applications that use the DGN format, such as Intergraph's Modular GIS Environment (MGE) solution, also use MSLinks.
In ArcMap, MSLink and MSCtlg values are displayed in the virtual attribute table. The external data that is referenced by them can be added to ArcMap and appended to the feature layer attribute table using the Join dialog box.