Organizing CAD Data In ArcGIS: Part 4/7 Unique Values
Seeing everything all at once can be confusing, especially if everything looks alike. The saying "I can't see the forest through the trees!" can be very relevant when using CAD files as GIS content.
If you consider an AutoCAD or Microstation file as a spatial database of features, then the ArcGIS CAD Feature Dataset could be considered a database VIEW of that spatial database. Like a tabular database that contains many tables and records of raw data, an AutoCAD or Microstation contains raw spatial data.
When you add a CAD feature class to ArcMap by using the Add Data Tool, you can browse the file system to find the blue folder CAD Feature Dataset representation of a CAD file that in turn contains the five standard ArcGIS CAD feature classes, POINT, POLYGON, POLYLINE, ANNOTATION and MULITPATCH. Because ArcGIS tools rely on a homogeneous data type, (all points or all lines or all polygons, etc…) the simplest level of organization is to sort the entities into feature classes based on this fundamental characteristic; geometry type.
The default symbology chosen by ArcGIS to display the Microstation or AutoCAD CAD feature classes added to ArcMap is like any other GIS feature class. ArcGIS will symbolize all the features in the feature layer with a single symbol selected at random. This monochromatic single symbol representation doesn’t provide any clues to the potential meaning of the CAD objects. Like any GIS feature layer you will want to symbolize the CAD feature class in a meaningful way to convey some cartographic message. With CAD feature layers, the CAD properties that the CAD file was drawn with originally provide an excellent set of attributes upon which you can symbolize the map.
I generally find that using the CAD COLOR property or the CAD LAYER property as the column upon which to perform a unique symbol rendering of the CAD file gives you the most distinction between all the different types of lines or polygons in a CAD feature class. If you have used a lot of AutoCAD BLOCKS or CELLS in your CAD file, you may want to consider using the block name stored in the TEXT field of the CAD feature attribute table, as the column value upon which you perform a unique value rendering of the layer.
To change the symbology of a CAD feature layer or any feature layer in ArcMap, I right click on the layer in the ArcMap table of contents and then select the properties tab. Select the Symbology Tab, from the under the Categories heading in the Show: pane, select the Unique Values option and then make your choice of COLOR, LAYER, TEXT etc… from the Values Fields pull down list.
The result of changing the symbology of the CAD feature layer will display the CAD features with different symbols based on the different CAD layers, or CAD colors or whatever distinguishing column you have selected.