Getting Multiple Uses from Drawings Using External References

You now know that you can insert a drawing file into the current file as a block. You can also insert a drawing file as an external reference. The difference between an xref and a block is that an xref does not actually become part of the drawing's database. Instead, an xref is "loaded" along with the current file at start-up time. It is as if AutoCAD were opening several drawings at once-the currently active file you specify when you start AutoCAD and any file inserted as an xref.

The unique feature of an xref is that any changes you make to the xref automatically appear in the current file containing the xref. You don't have to update the xref file manually as you do blocks. AutoCAD also notifies you if the xref file has been altered.

Another advantage of xref files is that since they do not actually become part of a drawing's database, the drawing size is kept to a minimum. This results in more efficient use of your hard disk space.

Xrefs are an excellent way to get multiple uses out of one drawing. You can create a floor plan drawing and then use it as an xref in another file to place the plan in a site plan. You can then use the file again for enlarged views of kitchen and bath plans and again for a roof plan or overall floor plan. Since you're not duplicating the file, you save on disk and RAM space. And since all these drawings are derived from the same source, you can change the original plan, and all its xref instances will show the same data. You don't have to update multiple drawings.

When a file is used as an xref, you can use most of the standard commands and tools on the xref with the exception of editing individual objects in it. You can modify its layer properties, turn layers on and off, scale, rotate, or move the xref, and you can use osnaps to locate geometry just like any other object in your drawing. If you need to make changes to the xref, you can either open it directly or use the Refedit command. You can issue Refedit by double-clicking an xref. New and casual users should stick to editing xrefs directly, because the Refedit command is a bit tricky.

You cannot xref a file if the file has the same name as a block in the current drawing. If this situation occurs but you still need to use the file as an xref, you can rename the block of the same name using the Rename command. You can also use Rename to change the name of various objects and named elements.

Attaching a Drawing as an Xref

You can attach as many files as xrefs as you want in any drawing. The process is fairly simple, as shown in the following steps.

The Open option in the Xref Manager is not available in LT.

1. Enter xr↲ or choose Insert è External References to open the External References palette (see Figure 8.13).

2. Click the Attach DWG button in the upper-left corner of the palette to open the Select Reference File dialog box. This is a typical AutoCAD file dialog box, complete with a preview.

3. Locate and select the drawing file you want to insert, and then click Open to open the External Reference dialog box (see Figure 8.14). Notice that this dialog box looks similar to the Insert dialog box. It offers the same options for insertion point, scale, and rotation.

4. Click OK to proceed with the xref insertion. If the Specify On-Screen option in the Insertion point group is turned on, you will be prompted to specify an insertion point; otherwise, the drawing is inserted in the default location, which is the origin of the current drawing.

Figure 8.13: The External References palette

Figure 8.14: The External Reference dialog box

In this example, you used the simplest default options in the External Reference dialog box. These default settings placed the xref in the drawing origin and used the default scale of 1:1 and a default rotation angle of 0. These settings place the xref in the current drawing as it would appear in the original xref file.

If you prefer, you can turn on the Specify On-Screen option for the Insert Point, Scale, and Rotation features, which in turn lets you set these values, either through the dialog box using the provided X, Y, Z, and Angle text boxes or in the drawing area using your cursor.

If you need to temporarily remove an xref from the current drawing, you can right-click the name of the xref in the Xref Manager palette and then select the Unload option. Right-click the name, and use the Reload option to restore an xref to the drawing.

Updating an Xref While You Draw

Since an xref is not actually part of the current drawing, other users can edit the source drawing of the xref even while you are editing the current, referencing drawing. When a file has been modified and saved while you have it open as an xref, the xref icon in the lower right of the AutoCAD window changes to show an exclamation mark. This alerts you to make changes in an xref in the current drawing. You might also see an alert message there.

When an xref is present in the current drawing and it has been modified, you can click the xref icon to open the External References palette. The xref that has been changed is indicated by a message in the Status column of the list box, as shown in Figure 8.15.

Figure 8.15: The message in the Status column of the External References palette shows that the xref needs to be reloaded.

You can then select the xref that needs to be updated and click the Reload option from the shortcut menu. You can also right-click a blank area of the External References palette and choose the Reload All Xrefs option from the shortcut menu to reload all xrefs in the drawing.

Differences between Xrefs and Blocks

Although blocks and xrefs are quite similar and can be used in similar ways, you'll want to keep in mind a few differences:

• Any new layers, text styles, or linetypes brought in with externally referenced files do not become part of the current file. If you want to import any of these items, use the Xbind command.

• If you make changes to the layers of an external-reference file, those changes are not retained when the file is saved, unless you check the Retain Changes to Xref Layers option in the Open and Save tab of the Options dialog box. You can find this option in the External References (Xrefs) section. This option instructs AutoCAD to remember any layer color or visibility settings from one editing session to the next. In the standard AutoCAD settings, this option is on by default.

• To segregate layers in xref files from layers in the current drawing, the xref file's layers are prefixed with their file's name. A vertical bar separates the filename prefix and the layer name when you view a list of layers in the Layer drop-down list or the Layer Properties Manager dialog box (as in Unitxref | wall).

• You cannot explode xrefs. You can, however, convert an xref into a block and then explode it. To do this, select the xref in the External References palette, right-click, and then choose Bind from the shortcut menu. You then have the option to bind the xref or insert it. If you choose Bind, the xref layer names are converted by changing the vertical bar into $0$. This keeps layers from the xref separated from the layers in the current drawing. The Insert option keeps the layer names from the xref in their original form. If an xref contains a layer with the same name as the referencing drawing, the two layers are merged.

• If an xref is renamed or moved to another location on your hard disk, AutoCAD won't be able to find that file when it opens other files to which the xref is attached. If this happens, you must use the Browse button, next to the Found At option at the bottom of the External References palette, to tell AutoCAD where to find the cross-referenced file. The Browse button appears to the far right of the option when you click the Found At input box.

Take care when relocating an xref file with the Found At option's Browse button. The Browse button can assign a file of a different name to an existing xref as a substitution.

Xref files are especially useful in workgroup environments in which several people are working on the same project. For example, one person might be updating several files that are inserted into a variety of other files. Using blocks, everyone in the workgroup must be notified of the changes and must update all the affected blocks in all the drawings that contained them. With xref files, however, the updating is automatic; and you avoid confusion about which files need their blocks updated.

NESTING XREFS AND USING OVERLAYS

You can nest xrefs; however, this can create problems with circular references. A circular reference is one in which the referencing drawing is actually referenced in an xref. To avoid this problem, use the Overlay option in the External Reference dialog box. If you insert an xref as an overlay, AutoCAD ignores any xrefs that might be attached to the xref you are importing into the current file.

You don't have to limit the use of the Overlay option to circular references. You can use it whenever you want AutoCAD to ignore nested xrefs.