AutoLISP Reference to Coordinate Systems provided by Autodesk.
World coordinate system - the reference coordinate system. All other coordinate systems are defined relative to the WCS, which never changes. Values measured relative to the WCS are stable across changes to other coordinate systems.
User coordinate system - the working coordinate system defined by the user to make drawing tasks easier. All points passed to AutoCAD commands, including those returned from AutoLISP routines and external functions, are points in the current UCS. As far as I know, all coordinates stored in DXF files are always WCS or OCS never UCS.
User defined coordinate systems are not just helpful for interactive CAD, therefore ezdxf provides a converter class
UCS to translate coordinates from UCS into WCS and vice versa, but always remember: store only
WCS or OCS coordinates in DXF files, because there is no method to determine which UCS was active or used to create UCS
Object coordinate system - coordinates relative to the object itself. These points are usually converted into the WCS, current UCS, or current DCS, according to the intended use of the object. Conversely, points must be translated into an OCS before they are written to the database. This is also known as the entity coordinate system.
Because ezdxf is just an interface to DXF, it does not automatically convert OCS into WCS, this is the domain of the user/application. And further more, the main goal of OCS is to place 2D elements in 3D space, this may be was useful in the beginning of CAD, I think nowadays this is an not often used feature, but I am not an AutoCAD user.
OCS differ from WCS only if extrusion != (0, 0, 1), convert OCS into WCS:
# circle is an DXF entity with extrusion != (0, 0, 1) ocs = circle.ocs() wcs_center = ocs.to_wcs(circle.dxf.center)
Display coordinate system - the coordinate system into which objects are transformed before they are displayed. The origin of the DCS is the point stored in the AutoCAD system variable TARGET, and its Z axis is the viewing direction. In other words, a viewport is always a plan view of its DCS. These coordinates can be used to determine where something will be displayed to the AutoCAD user.