Depending on what discipline you work in, you may or may not have the need to work with User Defined Coordinate Systems. By default, a new empty AutoCAD drawing starts out in the World Coordinate System (WCS) by which all objects are defined. You are free to define any other User Coordinate System (UCS) at any time.
If you have ever tried to copy an item from one drawing to another and the copy does not come in where you think it should, you might be dealing with a different UCS in each drawing. That is why it is important to leave the UCSICON visible so that you can easily tell at least some initial indicators about the UCS.
The two UCS icons above each represent WCS. The difference is explained below.
Above is the stock 2D UCSICON shown in WCS. We know it is WCS because of either the “W” or the square at the intersection of the X and Y axis. There are two different icons for WCS because you can choose to have the UCSICON visible as 2D or 3D. The dialog below can be found by running the UCSICON command and then “P” for properties.
The UCSICON has many user configurable properties.
As soon as the drawing is changed from WCS the UCSICON changes. Note the different varieties below.
The UCS has been rotated about the X axis very slightly (0.000001 degree) here.
The UCS has been rotated about the Z axis here.
The UCS has been rotated about the Y axis here.
The UCS icon here is visible during 3D operations such as 3DORBIT.
The UCS has been rotated about the X axis almost 90° so that the UCSICON almost disappears.
Because it is possible to change the UCS without actually affecting the display of the drawing, being able to quickly scan the UCSICON is important especially if you are working with other drawings of unknown origin.
How do you get a drawing back to WCS? Type in the UCS command and press ENTER. The default option for this command is always “World” so pressing ENTER will do the trick. The UCSICON will return to WCS, but the drawing may not change. If UCSFOLLOW = 0, then the drawing will stay put regardless of changes to the UCS. In this case run the PLAN command and press ENTER twice. This will reset your drawing back to WCS.
Lastly, you can also use the UCSICON command to control the placement of the UCSICON, optional colors for the UCSICON, and whether or not the UCSICON is a selectable object itself for rough, graphical UCS adjustments.
Here are some quick links to the AutoCAD 2015 docuemntation on UCSICON
- About Controlling the Display of the User Coordinate System Icon
- UCSICON (Command)
- UCSICON (System Variable)
