December 13, 2005

Semantic translation Part 6: The Meaning of Color

Symbolic variance in CAD is the most common way to differentiate between features. The combination of color, layer, linestyle, etc..., whether documented or not, compose the major part of a CAD drawing's codified CAD standard. When these combinations of CAD symbolic properties are known they can be leveraged to identify sets of features one from another, but can also be used to populated a feature classes' attribute table. Coded attributes can be extracted using a combination of ArcGIS queries tools like MAKE FEATURE LAYER, SELECT, etc... and then database tools like CALCULATE to populate a GIS database of attributes from information codified in the CAD symbology.

The CAD color of a line may denote the material type of a pipe. The meaning of the color RED can be documented by selecting all the converted pipes in a feature class that had the CAD color of RED,and then using a database tool like CALCULATE to change the pipe's MATERIAL field to the value of "IRON".Taking this concept one step further, you can use a look-up table. The look-up table can be a simple spreadsheet or database table with all of the possible CAD colors for pipes and another field that contains the "meaning" of those colors. That table could be joined temporarily to the feature class or the values can be transferred with a permanent joining of the table, or an operation that would copy the information into the feature class attribute table using a combination of ArcGIS tools like ADDJOIN and CALCULATE.

The point I am making here is subtly different from simply identifying features. A big part of semantic translation is not only identifying different features, but also understanding as much as possible about the CAD author's intended meaning. This meaning would include as many descriptive attributes as is needed in the GIS that may be hidden in symbology in the CAD file. This hidden information is often encoded in the CAD drawing according to symbolic or cartographic convention. Sometimes information is encoded by including CAD objects near, above, below or inside another entities. TEXT above a line may describe a feature's diameter attribute while a TEXT entity below a line may identify the feature's design length. Sometimes a feature has a different attribute when another feature is present, other times the omission of a companion object can translate into yet a different descriptive attribute.

Next time we'll examine a sample ModelBuilder model that includes some creative uses of spatial analysis tools to solve a specific workflow scenario. We'll take a look at how these ModelBuilder models are constructed and show through this one example, how you might use spatial tools in the semantic translation.

Continue to Part 7...


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