AutoCAD® 2002 - Autodesk

Extended data group codes and descriptions (continued)
Entity name Group code Description
Scale factor 1042 Also a real value that is scaled along with the parent. The difference
between a distance and a scale factor is application-defined
Integer 1070 A 16-bit integer (signed or unsigned)
Long 1071 A 32-bit signed (long) integer
Object Coordinate Systems (OCS)
To save space in the drawing database (and in the DXF file), the points
associated with each entity are expressed in terms of the entity’s own object
coordinate system (OCS). The OCS was referred to as ECS in previous releases
of AutoCAD. With OCS, the only additional information needed to describe
the entity’s position in 3D space is the 3D vector describing the Z axis of the
OCS, and the elevation value.
For a given Z axis (or extrusion) direction, there are an infinite number of
coordinate systems, defined by translating the origin in 3D space and by
rotating the X and Y axes around the Z axis. However, for the same Z axis
direction, there is only one OCS. It has the following properties:
■
■
Its origin coincides with the WCS origin.
The orientation of the X and Y axes within the XY plane are calculated in
an arbitrary but consistent manner. AutoCAD performs this calculation
using the arbitrary axis algorithm (see “Arbitrary Axis Algorithm” on page
175).
For some entities, the OCS is equivalent to the WCS and all points (DXF
groups 10–37) are expressed in world coordinates. See the following table.
Coordinate systems associated with an entity type
Entities
Notes
3D entities such as line, point,
3dface, 3D polyline, 3D vertex,
3D mesh, 3D mesh vertex
These entities do not lie in a particular plane. All points
are expressed in world coordinates. Of these entities,
only lines and points can be extruded. Their extrusion
direction can differ from the world Z axis
Object Coordinate Systems (OCS) | 173