NCS and the AIA CAD Layer Guidelines

Investigating an Implementation of the National CAD Standard (NCS) can be as simple as starting Civil 3D with one of the NCS templates that ships with Civil 3D.  Civil 3D 2009 includes three Imperial templates with layer naming conventions intended to mirror the AIA CAD Layer Guideline (Metric templates of the same name are included as well):

•    _AutoCAD Civil 3D (Imperial) NCS Base
•    _AutoCAD Civil 3D (Imperial) NCS Extended
•    _AutoCAD Civil 3D (Imperial) NCS LDT

In order to maintain “substantial conformance,” per the NCS, you need to follow the hierarchy as establish by the AIA CAD Layer Guidelines.  The hierarchy consists of five components:

1.    Discipline designator
2.    One major group
3.    Optional minor group
4.    Second optional minor group
5.    Optional status field

In the example above, the discipline designator AI refers to Architectural Interiors.  Major groups are intended to describe building or construction systems, in this case walls.  If you follow the NCS to the letter of the law, the major group layer names may not be modified, and user-defined major groups may not be introduced.  The first optional minor group is intended to describe different major group types (full wall, half wall, etc.), and the second optional minor group is intended to describe the major group even further.  The optional status field, in this case “N,” is a single character field that distinguishes the data contained on the layer according to the status of work or the construction  phase (in this case “N” identifies the status of work as being “New).

At first glance, some of the layers may not appear to conform to the NCS when taken literally.  In fact, there is only a single major group (group with a major designator) of layers in earlier releases of Civil 3D (not Civil 3D 2009) that don’t conform to a literal definition of the standard:  C-CORR*.

Before modifying layer names beginning with C-CORR, think of the number of times corridors need to be shared with outside sources.  Typically a corridor is created so that a surface representing proposed conditions can be extracted.  When this is the case, the corridor layers are a moot point – they are seldom shared.  If you decide (and it’s just too simple to ignore) to go with the standard, be careful not to get too wrapped up defining layers for objects that may never be shared.

Another thing to consider:  Corridors are built to graphically design or describe a feature or features, such as a road.  As C-ROAD is a proper name per the NCS (the Civil 3D 2009 templates use this as opposed to C-CORR, one can simply use the AutoCAD RENAME command to rename all corridor layers.  The RENAME command won’t rename layers established as object layers, however.  They will need to be renamed individually (but there are few of them).

Per the layer guidelines, the minor groups are optional (though the NCS has provided a list of recommended minor group names), and user-defined minor group fields are permitted.  They must contain four alphabetic and/or numeric characters and/or “~”, and must be fully documented on the NCS Compliance Disclosure Statement for the project on which they are used (read that again carefully, it effectively says – “free pass” to define layers to suit your needs, just be sure to follow the rules).  Therefore, with the exception of layers “Defpoints” and “0,” each of the layers found in the NCS templates, do indeed conform to the AIA CAD Layer Guidelines as found in the NCS.

Whether you’re in North or South America, or overseas, check out the AIA Layer guidelines (low cost) or the very similar A/E/C CAD Standard (no cost) if you’re looking to standardize.  Still at odds?  Check out the list of firms that have adopted the standard.

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