Controlling the User Coordinate System (UCS) in 3D

When you work in 2D space, you can shift the UCS to locate the origin point (0,0) closer to the area of the drawing in which you're working. You also can establish a new coordinate orientation when working with angled sections. In 3D, the UCS provides an additional benefit. You can use it to establish a new 3D plane in which to work.

A basic method for relocating a UCS is shown in Controlling the User Coordinate System (UCS). The following UCS command options are useful for relocating a UCS when you work in 3D space:

• Z AXIS. Defines a new UCS based on a new Z axis.
• 3 POINT. Specifies the new UCS origin and the direction of its positive X and Y axes.
• OBJECT. Realigns the coordinate system based on a plane established by an object you select. For objects with non-zero thickness, the Z axis matches the thickness direction, or extrusion.
• VIEW. Aligns the UCS XY plane perpendicular with the current view.
• X/Y/Z. Rotates the coordinate system around the axis you choose.

You may find that defining a different UCS to help you align the coordinate system with existing objects is easier than figuring out the exact placement of a 3D point.

AutoCAD LT keeps track of the last 10 UCS orientations specified in both model space and in paper space. You can save and restore UCS orientations by name, and you can also rename them.

Using UCS Presets

If you do not want to define your own UCS, you can choose from several preset coordinate systems. The image tiles in the UCS Orientation dialog box show the available choices. A colored line extends into one edge of the box on each image tile. This line represents the Z axis of the UCS represented by that tile.

If you have already specified a UCS, you can control whether the preset option shifts the UCS relative to the current UCS orientation or relative to the default World Coordinate System (WCS). The Relative to Current UCS and Absolute to WCS options have no effect if you restore the WCS, restore the previous UCS, or set the UCS to the current view.

Changing the UCS in Paper Space

You can define a new UCS in paper space as you can in model space; however, the UCS orientations in paper space are restricted to 2D manipulation. Although you can input 3D coordinate values in paper space, 3D viewing operations such as DVIEW, VPOINT, and PLAN are not available.

Changing the Default Elevation

The current elevation established by the ELEVATION system variable remains in effect as you change from one UCS to another, and it defines the drawing plane of the current UCS. AutoCAD LT maintains separate "current" elevation settings for paper space and model space. To avoid confusion, you can leave the elevation set to zero and control the XY plane of the current UCS with the UCS command.