Semantic Translation Part 3: Composite Features
Combining Apples and Oranges is perhaps not necessarily the best way to characterize GIS and CAD interoperability, unless you consider that what we are really trying to do is create a GIS fruit salad. If the science of databases is applied to the problem of GIS and CAD translation one would discover that these two different databases contain various rows and columns and related tables of information. These rows and columns are generally well defined on the GIS side. On the CAD side instead of a formal database we may consider them more like collections of spatial spreadsheets or like a CSV files of geometry that have one or more different attributing schemes as discussed in a previous post. CAD may not be a database, but you can make some assumptions and create a view of a drawing as a database table. (More about that in a future post.)
One very common and straight forward method of assembling mapping features with descriptive, or identifying attributes is to place a separate TEXT entity near, inside, or on the CAD entity it is intended to describe. Instead of some foreign-key that links the two spatial database records together, there is a relationship that is inferred based combinations of proximity, inclusion, orientation or intersection. For example a TEXT entity may be placed into a CAD drawing near the midpoint of a LINE entity that itself is intended to depict a pipe feature. The TEXT entity displays the diameter of the pipe. When the drawing is read as a map document someone knowledgeable about the map's content and cartographic expressions, sees the text near the line, and by convention understands that the meaning of the combination of the blue line and accompanying text is a water pipe of a certain diameter.
Using an object-by-object conversion that includes the TEXT entities and LINE entities would result in two separate autonomous objects. There are no digital links between the two separate objects that were placed by the drawing author. The text was placed with the intention that their relationship would be assumed and understood based on its proximity to the midpoint of a line.
As luck would have it, GIS software excels at spatial analysis and relationship building. Standard GIS tools like NEAR, SPATIAL JOIN's, INTERSECT, and IDENTITY provide the means to resolve these tyes of inferred relationship. Tools that find the mid-point of lines, like FEATURE TO POINT, or or POLYGON TO LINE also provide the means to be more specific about the orientation of objects one to another. You can see some of these ArcGIS tools in being applied within the CAD Translation Sample Toolbox.
Continue to Part 4...