Organizing CAD Data In ArcGIS: Part 1/7 Tools for CAD Data
I thought I’d try a provocative title for sweeps week. Or, how about another title… “Making Sense of this CAD Spaghetti, That Your Meatball Intern Has Drawn!" …Now I’m getting hungry.
The truth be told CAD data can be as smart or dumb as you want it to be. This sometime contributes to the problem, some CAD drawings are very organized and adhere to strict standards and logical data constructs. Other CAD drawings can be either ambiguous, or could be called “creative” at best. The science of reading CAD drawings in a GIS application is founded on the premise that CAD files are a collection of GIS features with various CAD properties that can be used to distinguish one from another. However it would be presumptuous for the GIS to ascribe meaning to the CAD objects (see series on Semantic Translation). Organizing CAD data by layers is good thing but so is using layers to define subtypes or entire data systems that might include points, line and area features.
One of the foundations of GIS is the fact that it stores simple standardized data structures and uses powerful spatial query and analysis tools to perform smart tasks. Therefore, it makes the most sense for GIS to view a CAD drawing as simple set of features with attributes and then use the powerful GIS toolbox on those features as if they were native GIS features. Even though a single CAD drawing can have the complexity of an ArcGIS Map document with hundreds of included features classes, ArcGIS sees a CAD drawing as a data set of only five feature classes. Initially the only criteria for differentiation is geometric type (POINT, POLYLINE, POLYGON, ANNOTATION and MULTIPATCH). All of the linear features of a CAD file can be accessed in ArcGIS through that CAD file’s CAD POLYLINE Featureclass. All CAD Points, Blocks/Cells and other point type CAD Entities in a CAD file can be accessed in ArcGIS using the CAD POINT Featureclass.
How you choose to organize the contents of the CAD Feature classes is up to you or rather up to the data constructs, standards and workflow task you are attempting to solve. For example, you may want to deal with only the Right of Way lines in a CAD file as a GIS dataset or maybe Parcel Block lines. However, you may also want to consider using the right of way lines, block lines and lot lines to build polygons to depict parcels from the CAD data. Choosing how you view and organize your CAD drawings may initially require more attention, but having the flexibility to apply a customizable organizing filter to your drawings provide additional problem solving capability.
The concept of abstraction is key to geographic information systems, feature type standardization is another. Using a points, lines and areas to depict geographic features allows the same tools that work on sewer systems, to work equally well on bicycle trails... cities and wells, airports and elephant breeding grounds.
This series will be dedicated to techniques to organize CAD data in ArcGIS to make it the most usable for various workflow tasks.