MSD: Leveraging CAD Standards
Instead of defining a whole new path of interoperability, the Mapping Specification for DWG, MSD, falls completely within the existing DWG file data constructs and file format to provide a means to let GIS information persist between ArcGIS and AutoCAD workflows. In ArcGIS 9.3 a small change was made to the way CAD files were read to look for a simple query and entity attributes encoded on the entity’s extension dictionary in a DWG file.
I often need to share basemap parcels and attributed infrastructure data from the enterprise GIS with private developers and design contractors that use simple AutoCAD. Using ArcGIS 9.3 these contractors can use the GIS information in plain AutoCAD. Previously the best I could do was create blocks with attributes as points and simple geometry. Sharing attributed lines and polygons was always awkward in plain AutoCAD.
In AutoCAD I can add information exported from ArcGIS. The GIS feature classes when exported create MSD feature class definitions and a copy of the GIS schema in AutoCAD for me to use. I am now able to populate attributes on standard CAD entities using that schema. Inside AutoCAD I can simply add to the data on the AutoCAD layer that is used as the MSD feature class definition criteria. If ArcGIS ever opens the DWG file again it will understand the new information as belonging to that MSD CAD feature class.
Feature Class groupings in MSD are based on a simple CAD property-based filtering query. It is very similar to the concept of a definition query on a CAD feature class added to ArcMap. Instead of creating data in a specific data or custom object somewhere in CAD, I define which entities in a CAD file participate in the feature class. Entities that satisfy the filtering criteria are considered part of the feature class.
I think it is best to use the most basic criteria to define the MSD groupings such as CAD layer names, however more complex criteria can be used. Since most CAD Standards use Layer names as the primary method of organizing data now, by defining an MSD feature class based on CAD layers or combinations of CAD Layers, I can build GIS-ready data with only minor changes to my existing CAD workflows. MSD feature classes are usable in an AutoCAD drawing either by adding the MSD feature class definition within AutoCAD, by adding entities to data exported from ArcGIS, or by utilizing a seed file with the feature class definitions already defined from either or both sources.