DGN Design Environment
In 2D drafting, the equivalent of a sheet of drafting paper is the design plane. Unlike a sheet of drafting paper, however, the design plane (or cube in 3D) in a DGN file is extremely large, letting you draw your models at full scale. To draw various elements in your model, you enter data points. Each data point placed in the design plane has associated X (horizontal) and Y (vertical) positions or coordinates. The design plane, then, is simply a Cartesian coordinate system upon which your model lies.
Coordinates are expressed in the form (X,Y). In the Seed files delivered with the product, the point called the Global Origin is set to the design plane's exact center and assigned the coordinates 0,0.
For most purposes the Global Origin in the provided seed files is fine. You may, however, wish to change the location or coordinates of the Global Origin. For example, an architect may want all coordinates to be positive values. A mapper or surveyor may need to use a grid system defined by a government agency to reference all locations to a specific point.
When you enter a data point, its coordinates are saved in IEEE 64–bit floating point format. The 3D Design cube is similar to the 2D design plane, but with a third axis Z (depth). Points in 2D models are stored as coordinate values expressed in the form (X,Y), while those for 3D models are stored as (X,Y,Z).