December 09, 2005

Semantic Translation Part 4: CAD Direct-Read

( P ) Pronunciation Key (s-mntk) also se·man·ti·cal (-t-kl)adj.
Of or relating to meaning, especially meaning in language.
Of, relating to, or according to the science of semantics.

( P ) Pronunciation Key (trns-lshn, trnz-)n.
The act or process of translating, especially from one language into another.
The state of being translated.

In this nerdy topic, being specific is necessary to communicate my meaning. We've talked about the fact that conversion or translation without special attention to meaning is only partially useful. It is good to define the problem, but solutions are more useful! In the previous post we talked about one solution to inferred spatial relationships being handled with GIS tools that have a spatial awareness and can provide usable results to questions like, "what is close to, inside or around this object?" GIS tools can be used to codify the rules for translation. In written and spoken languages we have grammar, sentence structure and figures of speech that can be defined to some extent as rules. The rules can be expressed in terms of word definitions, conjugation, word order and then loosely by phrases that form figures of speech that are arguably harder to codify. Establishing a framework for building understanding is necessary to perform the task that takes data, applies rules and creates meaning or information. The first step in this process is to understand the words of the language to be translated.

In the case of CAD to GIS conversation the GIS must understand the CAD expressions; its geometry, properties and various forms of extended attribution. Like the translation of Chinese to English, or any language from one to another, a critical first step is understanding Chinese words. There are two major schools of thought in this first process. One is to convert one language to a middle language and from that language convert to the destination language. This has its benefits in that ideally every language would just need to be converted to a single other language and likewise from one other language, (easier said than done, but still an interesting approach). This approach creates some intermediate format, like Chinese to French, and then French to English. The richer the intermediate format the more complex the initial conversion might be, but the better the translation can be overall.

The other method is to convert the base language directly into the words of the target language. This has distinct advantages, especially if you are only concerned with translating to and from your language. A direct read can help you avoid the "Chinese telephone" problem where differences in word subtleties may be compounded by orders of magnitude from language to language or just from multiple translations. For those of you who read English as a second language and especially if you are Chinese, I apologize, but this too is an example of the problems of semantic translation since the figure of speech "Chinese telephone" does have a very specific meaning to most in the United States familiar with the child's game of the same name.

A direct read of CAD data by ArcGIS is an on-the-fly conversion. ArcGIS sees CAD files as collections of ArcGIS feature classes not as CAD drawings. On the disk they are still CAD files, but ArcGIS doesn't see them that way. ArcGIS sees CAD files as collections of GIS objects that have a table of attributes. CAD files are not stored as feature classes with tables of attributes, but ArcGIS builds a view of the CAD file in memory where it applies assumptions and applies constraints to the CAD data to turn it directly into GIS data. As discussed in a previous post this GIS abstraction of the CAD data makes it directly usable by GIS tools. The reliable and predictable conversion of CAD data to GIS data is performed by the direct read capability, provides the foundation of translation. This GIS view of the CAD data can then be used as input to GIS tools and processes to codify rules-based semantic translation in the GIS language.

Continue to Part 5...


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