CAD systems offer various export formats for the data exchange with other programs. One of these is the IGES format.

IGES is the abbreviation for Initial Graphics Exchange Standard and was standardized by the American National Standards Institute (ANSI). The IGES format is independent of any vendor  or his software. This allows the design engineers to transfer their design data via the intermediate step of IGES from a CAD system (e.g. CATIA) to another CAD system (e.g. I-DEAS) or a graphics program (e.g. IsoDraw, IsoDraw CADprocess), as visualized in the following diagram:

IGES is widespread in the CAD world. It can describe 2D and 3D data. It focuses on the precise transfer of design details while its graphic options are limited. Apart from the common graphic primitives such as lines or circles, also complex elements like surfaces, volume bodies or measures, compound elements and perspective parameters.

The single elements within an IGES file are described through so-called entities. There  are a great number of entities: Entities can describe circles, lines, text etc., but also structures like groups and links to existing elements. IGES files are  built up in plain text. This means you can open and read the files with an editor.

Under a header you can find two sections in which the entities are listed. In the first section they are listed with special parameters and they link to the second section where the entities are defined.

A typical data description through the entities in the second section can look thus:

102,4,519,521,525,527;	529P	476
142,1,507,517,529,2;	531P	477
144,507,1,0,531;	533P	478
128,2,2,2,2,0,0,0,0,0,0.0,0.0,0.0,1.0,1.0,1.0,0.0,0.0,0.0,1.0,	535P	479
...

In the above example the following entities describe the elements:

102:	Composite Curve Entity
142:	Curve on a Parametric Surface Entity
144:	Trimmed Parametric Surface Entity
128:	Rational B- Spline Surface Entity

A description of the IGES standard can be found on the following website: https://www.uspro.org.

Generally, you do not need any specific knowledge about the content of IGES files in order to be able to exchange this data via this format.

When reading an IGES file with IsoDraw CADprocess, the entities are interpreted and transformed to drawing elements. An element can be defined in different ways what can result in communication difficulties between the applications. Often, the lack of a universal description(s) plus control of the quality of an IGES file may result in major losses or even impede the transfer as a whole.

In order to decrease this danger there are efforts to limit the volume of the IGES standard by defining subsets which can be processed according to specified conventions. In this context especially two terms are important: VDAIS und CALS.

The Association of German Car Manufacturers (Verband der deutschen Automobilindustrie (VDA)) has defined various groups of elements, which must be interpreted in a precisely defined manner. Thus, a program that can read and write IGES according to the VDAIS specification, must also name the group to make clear which complexity of the file can be expected from the program. On the other hand, IGES files created from this program must be limited to the elements that are defined in the declared group.

The intent of the CALS specification is similar: It defines 3 classes that determine the volume as well as the method of interpretation. The first class relates to Technical Illustration, the second to design drawings and the third to electric/electronic applications.