How to Use User Coordinate Systems

Quick answer: Every drawing has a fixed World Coordinate System (WCS), which can’t be deleted or modified. A User Coordinate System (UCS) is one you define yourself, anywhere in 3D space, and you can create, save, and restore as many as you need. New entities are created in the current coordinate system’s XY-plane by default, controlled by ELEVATION.

WCS vs UCS

ELEVATION sets the distance from the current coordinate system’s XY-plane; at its default of 0, entities are created directly in that plane. A new drawing starts with WCS as the current coordinate system, shown by the letter “W” in the UCS icon. In plan view, the icon is seen from directly above, with the Z-axis pointing straight at you; in other views, it reflects your current viewpoint instead.

The UCS Icon

Colour-codes the three axes: X is red, Y is green, Z is blue, controlled by COLORX, COLORY, and COLORZ respectively. Only the visible portions of the axes represent the positive direction.

Two variables control its display:

• UCSICON: has a “Show icon” option (on/off), and an “At origin” option. With “At origin” on, the icon marks the current coordinate system’s actual origin, unless that origin falls outside the viewport, in which case it moves to whichever corner UCSICONPOS defines instead. With “At origin” off, it always displays in that corner regardless of where the origin actually is.
• UCSICONPOS: defines which corner of the viewport the icon uses when it’s not shown at the origin.

Automatic UCS Behavior

• UCSORTHO: when on, an orthographic UCS restores automatically whenever a matching orthographic view is restored, whether via the -VIEW command or selected on the LookFrom control. Its origin sits at either the WCS origin or the UCS referenced by UCSBASE. The six orthographic views are top, bottom, left, right, front, and back.
• UCSVP: with multiple viewports open, on keeps each viewport’s UCS fixed independently; off makes every viewport follow the UCS of whichever one is currently active.
• UCSFOLLOW: when on, the plan view (zoomed to extents) generates automatically any time the coordinate system changes.
• PLAN: restores the plan (top) view of the current coordinate system, UCS or WCS.

Defining a UCS

1. Launch UCS.
2. Specify the origin point.
3. Specify a point to define the positive X-axis.
4. Specify a point to define the positive Y-axis. The UCS is now defined.

Defining and Saving a Named UCS

1. Launch EXPUCS. The Drawing Explorer opens to Coordinate Systems.
2. Click New. The dialog temporarily closes so you can define the new coordinate system.
3. Choose the 3 point option at the Command line.
4. Specify the origin point.
5. Specify a point for the positive X-axis.
6. Specify a point for the positive Y-axis. The UCS is defined, and the Drawing Explorer reopens.
7. Click the new UCS’s Name field and replace the default <NewUCS> with something meaningful.
8. Close the Drawing Explorer.

Restoring a Saved UCS

1. Launch EXPUCS.
2. Click the blank tile in front of the UCS you want, to make it current.
3. Close the Drawing Explorer.

Restoring the WCS

Launch UCS and choose the World option.

Frequently Asked Questions

Can I delete or modify the World Coordinate System? No, the WCS is fixed and can’t be deleted or modified. You can create and modify as many UCSs as you like instead.

What colour represents which axis on the UCS icon? Red for X, green for Y, blue for Z, controlled by COLORX, COLORY, and COLORZ.

Does the UCS icon always show at the coordinate system’s actual origin? Only if UCSICON‘s “At origin” option is on, and even then, only if that origin is currently visible within the viewport. Otherwise it displays in the corner set by UCSICONPOS.

How does the LookFrom control relate to UCS? If UCSORTHO is on, selecting an orthographic view on the LookFrom control (or restoring one via -VIEW) automatically restores the matching orthographic UCS too.