The Mysteries of AutoCAD Object Data: Part 3
Autodesk Map 3D is a collection add-on customization code to extend the data structures of the basic CAD data model of AutoCAD to support the attribution of its geometric primitives for use in mapping. Autodesk 3D Civil is the same type of application to extend AutoCAD for Civil Engineering. Their primary method of attribute storage is object data. Autodesk Map/Civil 3D cannot function solely with standard CAD graphic entities. They introduces more sophisticated GIS data structures in the form of AutoCAD custom objects.
The definitions of the AutoCAD custom objects are stored in the AutoCAD data dictionary like object data, in some ways these custom objects are like a block definition that is stored in the AutoCAD symbol tables. However like the x-records used to store object data the custom objects and their complex geometries are not discernable without the controlling application that created them. To manipulate these custom objects AutoCAD must be re-educated on how draw and manipulate these foreign data structures. It is the responsibility of the controlling application to instruct AutoCAD what the various methods and properties are that will be supported by the custom object.
Neither AutoCAD nor LT can understand these objects. Read/Only versions of certain applications called object enablers can be obtained by AutoCAD users to understand these complex objects and sometimes their attribute data. The other method of making a custom object more readable to applications like AutoCAD and AutoCAD LT is for the custom application to create a proxy geometry definition along with the custom object. ArcGIS 9.2 now reads proxy geometries of AutoCAD custom objects as GIS features. A proxy geometry may be a pretty good representation of the original geometry, without its clever behaviors and attribution or it may be something less than that, all depending on the application that defines it.