BACKGROUND CAD-techniques

The use of CAD-techniques in building design has increased rapidly during last 10 years and is today common practice for producing building documentation. As a consequence of this, the need to transfer CAD-information between the different participants in a construction project in digital form, and not only as plotted paper drawings, has become of vital importance. In contrast to the layout and symbols of paper drawings, which in most countries is more or less standardised, the techniques for managing digital CAD-data are still in their infancy. A representative of a major Swedish design company recently jokingly remarked, that in major projects the specifications for information co-ordination now seem to be more voluminous that the design specifications themselves. This remark is a clear symptom of the problems caused by the lack of standard data structures for information management in integrated CAD design.

 The transfer between CAD-systems of the graphics contained in output drawings alone, which to some extent can be handled using standards such as the DXF-format, is not enough. Increasingly CAD-systems are used not as digital drawing-boards, but for managing integrated 2-D (or at best 3D) models of a complete building. (Excellent guidelines for end users and application developers have for instance been produced in Denmark (Abb 1993)). A system such as AutoCAD makes a clear distinction between model-space (containing the model of the building in world coordinates) and paper-space (containing output from such models in drawing sheet coordinates). As a consequence a prerequisite for efficient data transfer and sharing is that the information in such models must be structured and partitioned in standardised ways. In current CAD-practice quite elaborate layering schemes, often used in combination with the reference-file technique, provide the dominating method used to achieve this end.

In layering systems each drawing primitive is assigned to some layer. The user can then interactively decide which layers to show actively on the screen or to output on a plotter. 

Layering techniques were taken into use in the beginning of the 1980's (Port 1984). Early systems contained a limited number of layers ( typically 64 or 256), but nowadays the flexibility is greater. Different CAD-vendors have implemented layers slightly differently but the basic ideas are the same. The benefits of allocating different layers to the different design subdisciplines became evident early on. In very crude layering schemes the architect was for instance given layers 0-99 to work with, the structural engineer 100-199 etc. More sophisticated systems can be based on using national building element tables as a basis of the layer division.

User groups for particular CAD systems, individual bigger projects, large companies etc. have defined their own layering standards. Since the end of the 1980's national standards or guidelines had been developed in a number of countries (NSF 1992) , (AIA 1990), (BSI 1990). In some countries (i.e., Sweden) the market dominance of particular CAD applications for building design has provided de-facto standards. Although most of these standards seem to use quite similar basic principles for layer division, the implementations and syntaxes vary a lot and make data exchange difficult. Many of these standards also suffer from technical deficiencies resulting from the ad-hoc and incremental fashion in which they were developed.

This is the background for the decision of the committee TC10/SC8 of the International Organization for Standardization to appoint a new working group ISO TC10/SC8/WG13 with the scope of defining an international standard for the use of layering in construction (ISO 1990). The committee had its first meeting in Stockholm in October 1993 and a Draft International Standard was approved in September 1996 (ISO 1996 a,b), (ISO/TC10/SC8/WG13 1996).