The data structures that may be used in CAD systems to store the model are the same structures on which the model creation and manipulation algorithms act.

Techniques for model storage
Five techniques are used:
1. A data structure for interactive modeling using 2D and 3D wire-frame and surface geometry, where the relationships between geometric entities are less crucial than in solid geometry.
2. The storage of image vectors in a display file.
3. The association of geometric entities with those used in their construction associativity between entities.
4. The association of non-geometric data with geometric model through the use of attributes.
5. The collection of design models into a database and in particular the use of engineering data management systems.

Data structures for interactive modeling
First step: determine the specification for a data structure to support interactive modeling.
How? From the requirements imposed by the system, for which the structure, for example:
Allow interactive manipulation (addition, modification and deletion) of data. Support multiple types of data element (geometric, textual, dimensions, and so on. Allow properties such as pen number, line-style, color and so on to be associated with geometric elements. Be compact to minimize disk storage and main memory requirements. Allow models of various sizes and Provide as efficient an access to the data as possible.

These requirements between them pose significant constraints on the design of the data structure. For example, each entity type will in general require different amounts of data. In principle, storage could be allocated in a separate array for each entity type.

Display files
Two interactive operations that carried out in CAD:
*Redrawing of the display to “clean-up” unwanted clutter on the screen or to restore parts of the image that have been corrupted.
*Selection of entities from the screen.
In each of these cases, there is some merit in working not with the base entity data, but with a display file that stores the displayable vectors for an entity. For example, in the selection of a surface without use of a display file it would be necessary to recompute the surface display in order to identify which path is nearest to the cursor. With a display file it is only necessary to find the nearest displayed vector and to cross-reference back to the entity.

Display files also lend themselves to fast image manipulation or zoom facilities.
Instead of recomputing the entire image for a zoom within existing window boundary, the display file vectors are used- leading to some loss in display resolution for curves, but generally faster display control.
The display file is related to the rest of the data by cross-referencing between the display file and the entity table.

Integrated database
How to use standard database programs to handle the data? How to avoid duplication of data within the database?
An assembly model will be a collection of references to component models elsewhere in the database, together with the appropriate transformations to orient and place the models in the assembly.
Example of integrated database systems: Project-oriented database organization (See the Figure)
The system file-store is divided into a number of project areas and “standards” areas for standard component data. A particular assembly draws on models from within its project area, but also from the archive of another project.

Engineering Data Management System EDMS
A system to assist a design team in the indexing and search of its design data may be termed an engineering data management system EDMS.
The EDMS should:
Index and manage engineering design data, provide mechanisms to ensure that data is secure, but is available and easy accessible to all who need access, and also ensure that a particular data item is unique.
EDMS are often built using commercial database management system DBMS programs.
The more comprehensive EDMS are used to manage data across multiple computing systems to allow files to be transferred between a CAD system based on a network of workstations and mainframe computer used for archiving (See the following Figure).

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