Organizing CAD Data In ArcGIS: Part 3/7 Entities as Features
My father is a retired forester. He told me of a comment a surveyor friend once made. This surveyor told my father there are only two kinds of trees…stick trees and Christmas trees! As far as this surveyor was concerned that was all that mattered when identifying a tree. The only important distinguishing feature worth noting was whether it looked like a Christmas tree, or not. When ArcGIS sorts CAD entities in a drawing a similar albeit more useful categorization occurs.
ArcGIS doesn’t attempt to present CAD data using its free form CAD data structures, instead it overlays a standard GIS spatial database schema on the CAD file. CAD entities are sorted into different GIS feature classes and are thus recast as GIS features. By recasting the CAD entities as GIS features CAD data becomes a true GIS data source. All of the tools in ArcGIS designed to work with GIS data, work with CAD data because the first thing ArcGIS does when reading a CAD file is to organize the data into an ArcGIS CAD feature dataset of 5 standard feature classes (POINTS, POLYLINES, POLYGONS, ANNOTATION and MULIPATCH).
The resulting CAD POINT feature class will be built from all the CAD entities
and in the CAD file that have a single coordinate as their primary defining geometry. This would include AutoCAD BLOCK INSERTS, POINTS and SHAPE entities or Microstation CELLS, POINTS, etc… In the case of BLOCK INSERTS/CELLS there may be sub entities that are also sorted into other feature classes.
The CAD POLYLINE feature class will contain all linear entities and sub entities along with their true curves geometry; ARCS, LINES, POLYLINES, 3DPOLYINES, LWPOLYLINES, LINE STRINGS, etc… This feature class will also contain closed shapes such as TRACES, CIRCLES and closed POLYLINES.
POLYGONS are built from all closed shapes. These closed shapes will also be represented in the POLYLINE feature class so that you have the ability to treat closed shapes as loops or as areas. These CAD entitles would include CAD polygons, closed POLYLINES, TRACES, ELLIPSES, CIRCLES, etc…
The CAD MULTIPATCH feature class is comprised of CAD 3DFACES. These special ArcGIS polygon features are visible in 3D using ArcGlobe or ArcScene.
All CAD TEXT entities are included in the CAD ANNOTATION feature class. Their insertion point is uses as their primary geometry. The ArcGIS CAD ANNOTATION feature class can be used as POINT input to may ArcGIS tools.
All CAD feature classes have a standardized table of attributes. This virtual read/only attribute table is assembled from the CAD graphic properties of the CAD entities (COLOR, LINESTYLE, THICHNESS, LINEWEIGHT, etc...). This table also includes any Microstation TAG values and AutoCAD BLOCK ATTRIBUTE values. Using information in the table you can query and further sort the data.