Prepared by The AIA Task Force on CAD Layer Guidelines
Michael Schley, AIA, Chairman
Ken Sanders, Richard Buday, Dana C. Smith, David Takesuye
In 1990, the Task Force on CAD Layer Guidelines envisioned a future where the use of Computer Aided Design (CAD) would become commonplace and the sharing of graphic information routine. Six years later, this projected future has become a reality. The construction documents of most large projects are drawn electronically and an increasing number of facilities are being managed with CAD.
The Task Force on CAD Layer Guidelines was a collaboration of four professional associations and three government agencies. The professional associations were the American Institute of Architects, The International Facility Management Association, The Consulting Engineers Council, and The American Society of Civil Engineers. The government agencies were the Naval Facilities Engineering Command, The US Army Corps of Engineers, and the Department of Veteran Affairs. In 1989, the task force released a public draft for CAD Layer Guidelines. After reviewing comments from over five hundred individuals, the guidelines were finalized and published by AIA Press in May, 1990. Since then, numerous companies and agencies have adopted the guidelines for their use, making the document the de-facto standard for CAD use in architecture, building engineering, and facility management
In 1994, the AIA convened a small working group to study needs for updating of the document. The task group worked closely with committees at the Construction Specifications Institute, the National Institute of Building Sciences, the Tri-service CAD/GIS center, and the International Standards Organization to ensure compatibility with other emerging CAD standards.
This document represents a draft. As with the original guidelines, it is subject to public review before finalization and publication.
The Need for Revision and Improvement
The CAD Layer Guidelines published in 1990 were intended as the first step in an ongoing process. Because the guidelines relate to an area of technology that continues to change, it is important that the guidelines improve and evolve.
During the analysis phase, the need for several changes was identified:
Compatibility with new international standards for CAD organization.
Expansion of the discipline designators to accommodate interior design, telecommunications, and other fields. Guidelines for the use of reference files.
Coordination with the National CADD Standard being developed by the National Institute of Building Sciences.
Coordination with the Uniform Drawing System being developed by the Construction Specifications Institute.
Coordination with layer standards implemented by the U.S. Department of Defense Tri-Service CADD/GIS Technology Center. Simplified methods for indicating drawing annotation.
The Role of Layers in CAD
The Layer is the basic tool used in CAD for managing visual information. By accommodating the reuse of information, layers reduce drawing time and improve project coordination.
Although layers can be used to carry non-graphic data, there are more appropriate tools than layers for performing this function.
Conversely, one might wonder if new techniques using class libraries and object oriented data make layers obsolete. An analysis of these new approaches to CAD data shows that layers and object intelligence are complementary rather than competing technologies. Class libraries and object data are the appropriate tools for carrying non-graphic data. Layers are the appropriate tools for managing the visual aspects of graphic entities.
International Standards Issues
In February 1996, the International Standards Organization (ISO) published its proposed standard for CAD layer names. Balloting on the standard continues through July of 1996.
The ISO format starts with an originator code, followed by a six character code indicating building element. Each country is free to propose its own classification system in this portion of the layer name. The third field of the format is a two character annotation code. Finally, a number of other optional fields are defined for presentation, status, phase, view, scale, and work section.
Although building construction is a traditionally a localized industry, the world is becoming increasingly interrelated. The CAD Layer Guidelines task force believes that the interests of architects, designers, engineers, and facility managers in North America are best served by aligning our standards with those used internationally. This will avoid the specter of US and Canadian design firms being placed at a competitive disadvantage because of conflicting standards.
Without question, changing the layer format from the 1990 guidelines to the new short format will entail some amount of work. To reduce the inconvenience, the task force plans to provide public domain software utilities to assist with layer conversion. Weighing the cost of a one time conversion to the long term implications of multiple standards internationally, we believe that a switch to the new format is worthwhile.
Basic Organizing Concepts
The CAD Guidelines encompass organizing structures for two essential aspects of CAD data:
Files serve as the basic package containing graphic information. CAD files can be referenced by other CAD files to provide means of sharing data.
Layers are the principle means of categorizing graphic data within CAD files. By turning layers on or off, different views or plots may be defined.
The principle of agent responsible is followed in both file names and layer names. The agent responsible code provides a path back to the originator of the data, facilitating periodic updates. The code also provides a logical categorization.
The Agent Responsible code is a two character field with the second character either a hyphen or a user defined modifier. Defined codes for Agent Responsible are:
C Civil Engineering
F Fire Protection
G GIS (Geographic Information Systems)
I Interior Design
L Landscape Architecture
O Other Consultants
Q Equipment Consultants
Z Contractor/ Shop Drawings
A second character can be used to further define the agent responsible. For example, AG can be used for architectural graphics, QK for Kitchen Equipment consultants, and OH for Hazardous Materials consultants.
Two distinct types of CAD files are defined: models and sheets.
A model describes a subset of a building’s geometry and its physical components: walls, doors, windows, columns, beams, outlets, ducts, etc. A model has no scale; it is drawn life-size. Most buildings are described by a series of two-dimensional models – plans, elevations, sections, details – although the organizational concept also supports three-dimensional models. Models can be constructed heirarchially; in other words, models can contain other models.
A sheet contains one or more scaled views of one or more models arranged within a border and title block. Each sheet represents one – and only one – plotted drawing. Sheets are always plotted at full scale (1=1), and the origin of each sheet is at the lower left hand corner of the sheet border. Basically, a sheet is a ‘ready-to-plot’ CAD file.
Using a conventional database analogy, the models are the data and the sheets are the reports of the data.
Distinguishing between models and sheets is useful for two reasons. First, CAD drawings are often developed during design, long before the organization and layout of construction documents is known. Distinguishing between models and sheets allows design models to be developed early without concern for the specific layout of construction drawings to follow. Sheets representing the construction documents can be developed at a later stage, once the contents of the construction set are better understood, and the sheets can be modified at any time independently of the models they reference.
In addition, sheets provide an important mechanism to bring stability and order to the flexible and dynamic nature of CAD information. It is relatively easy to create different types of plots from the same CAD file, for example, using different scales, views, orientations, layer filters and pen/color settings. This flexibility – clearly one of CAD’s major blessings, especially during design phases – can become a curse during the construction documents phase. In terms of content, a plotted construction drawing needs to be more like a hand-drafted drawing: stable and predictable. Plotting anomalies must be carefully avoided. A sheet allows the user to encapsulate the parameters important to producing a successful plot – such as scale, layer visibility, and graphic appearance – within the sheet itself, without affecting the flexibility of the models it references.
Multiple sheets can be created using the same model(s), where each sheet contains a different graphic representation of the model(s) using different plotting parameters. In fact, a set of presentation sheets and a set of construction document sheets can be set up simultaneously using the same models.
In many cases, such as floor plans, ceiling plans, and enlarged plans, models and sheets are contained in separate files (see illustration). For example, a floor plan model, representing a single floor plan in a multi-story building, can be referenced to the overall floor plan sheet, the reflected ceiling plan sheet, and other enlarged plan sheets, such as lobby, stair, or toilet room sheets. Dynamic links between the models and sheets (updated automatically by the CAD system) can be established using reference files. Static links (updated manually by the user) can be established using blocks, cells, or symbols, depending on the features supported by the CAD system.
In some cases, models and sheets coexist in the same CAD file. Examples include interior elevations, details, or other drawings which contain no model data shared by any other drawing. For many small projects, it is not worth the effort to separate a model and sheet into two separate CAD files; both are easier to manage when they are contained in a single file.
In general, data that will be shared between multiple sheets should be contained in separate model files. A useful rule of thumb: models are always referenced by other files, while sheets are never referenced by other files.
Annotation can be placed in either models or sheets. Some annotation, such as dimensions, notes, and targets, are generally easier to coordinate and revise when they are included in the model. In fact, models may contain multiple sets of annotation intended for multiple sheets, such as sets of dimensions which appear on sheets at different scales. Other annotation, such as drawing titles, legends, and sheet-specific notes, are generally more convenient to include on the sheet itself. The layer guideline supports both annotation methods.
File Naming Guideline
A naming guideline is defined for both models and sheets. Sheet file names start with the agent responsible, followed by a user-definable numerical field which closely corresponds to the sheet sequence number. Model file names start with the agent responsible, followed by a two-letter drawing type, followed by a user-definable field.
File Naming Conventions
Naming conventions are defined for two types of CAD files:
Model files start with the agent responsible code followed by a two letter drawing type, followed by a sequential number. In order to accommodate an eight character file name, project designations are not included in the file name. Directories or folders should be used to separate projects.
Designations applying to all disciplines:
*-FP Floor Plan
*-SP Site Plan
*-DP Demolition Plan
*-QP Equipment Plan
*-XP Existing Plan
File Designators Specific to Each Discipline
A-EP Enlarged Plans
A-CP Ceiling Plans
A-RP Furniture Plans
A-NP Finish Plans
I-EP Enlarged Plans
I-CP Ceiling Plans
I-RP Furniture Plans
I-NP Finish Plans
S-FP Framing Plans
S-NP Foundation Plans
M-HP HVAC Ductwork Plans
M-PP Piping Plans
M-CP Control Plans
P-PP Plumbing Plan
F-KP Sprinkler Plan
*-VP Evacuation Plan (may be by other discipline)
C-RP Roads/ topo
Using numerical descriptors for sheet file names and alphabetical descriptors for model file names allows a person to distinguish between the two types of files by simply inspecting their file names. In order to support eight-character file names, project designations are not included within model or sheet files names; instead, directories or folders are used to separate project files.
Layer Name Formats
The Layer Guidelines are organized as a hierarchy. This accommodates expansion and user defined extensions to the layer list. Layer names are alphanumeric and use easy to remember abbreviations.
Two layer name formats are defined. The short format has been revised to be consistent with the ISO CAD standards. The long format is compatible with the long format in the 1990 guidelines.
The new short format is recommended. It is less cryptic than the original short format and is compatible with new international CAD standards. The long format is acceptable and may be preferred by some users.
___ ___ ___ ___ ___ ___ ___ – ___
^___^ ^_______^ ^___^ ^
| | | |
| | | Annotation
| | Minor Group
| Major Group
___ ___ ___ ___ ___ ___ – ___ ___ ___ ___
^___^ ^___________^ ^___________^
| | |
| | Minor Group/Annotation
| Major Group
The Agent Responsible code replaces the Major Group code defined in the 1990 guideline. This code is used to indicate the origin of the graphic information and provide a means of updating information. The field is a two character field with the second character either a hyphen or a user defined modifier. Defined codes for Agent Responsible are the same for both layers and file names.
Major groups identify the building system. Although major groups are logically grouped with specific agents responsible, it is possible to combine major group codes with any of the agent codes.
In the new short format, major groups use three characters. Examples are “WAL” for wall, “DOR” for door and “FLR” for floor.
This is an optional, two character field for further differentiation of major groups. For example A-WALPR indicating architecture, wall, partial height.
The following common modifiers are defined for use in the minor group field
DE To be demolished
EX Existing to Remain
NW New Work
This is an optional, one character field following a hyphen separator. Standard annotation codes are as follows:
B Borders and Title Blocks
C Construction Lines and non-plotting information
P Plot information
L Legends and Schedules