Going External with References in AutoCAD 2006

In AutoCAD, an xref, or external reference, is a reference to another, external file — one outside the current drawing — that you can make act as though it's part of your drawing. Technically, a reference is simply a pointer from one file to another. The xref is the actual pointer, but many people call the combination of the pointer and the external file the xref.

Drawings that you include as xrefs in other drawings often are called child drawings. Drawings that contain pointers to the child drawings are called parent drawings. This family terminology gets a little weird when you realize that a child drawing can have many parent drawings that refer to it — apparently it's the commune version of family relations. If you find such relationships odd, you can, like the AutoCAD online help system, refer to the parent drawing as the host drawing. The terms parent and child are preferred, in part because they're easily extendable to describing more complex hierarchies, such as a parent drawing, which xrefs a child drawing, which in turn xrefs a grandchild drawing.

Xrefs have a big advantage over blocks: If you change a child drawing, AutoCAD automatically loads the change into all the parent drawings that reference the child drawing.

AutoCAD loads all xrefs into the parent drawing each time the parent drawing is opened. If the child drawing has been changed, AutoCAD automatically incorporates those changes into the parent drawing.

When you open a drawing containing xrefs, AutoCAD displays a little symbol (which looks like papers with a binder clip) on the right end of the status bar. This symbol alerts you to the fact that some of the things you see in the drawing are actually parts of other, xrefed drawings. If an xref changes while you have the parent drawing open (because you or someone else opens and saves the child drawing), the status bar xref symbol displays an External Reference Files Have Changed balloon notification. (If you want to change whether the notifications appear and how often AutoCAD checks for changes, look up XREFNOTIFY and XNOTIFYTIME in the online help.) You can use the Reload option in the Xref Manager dialog box to show the updated xrefs.

The process of updating xrefs is simplified in AutoCAD 2006. The balloon notification now contains a Reload Modified Xrefs hyperlink. However, you can still click the status bar icon and run Xref Manager if you want to choose which xrefs get reloaded.

Another advantage of xrefs over blocks is that their contents aren't stored in your drawing even once. The disk storage space taken up by the original drawing (that is, the xref) isn't duplicated, no matter how many parent drawings reference it. This characteristic makes xrefs much more efficient than blocks for larger drawings that are reused several times.

You can always buy a larger hard drive, however, so the storage issue isn't crucial. The key benefit of xrefs is that they enable you to organize your drawings in a modular way so that changes you make to a single drawing file automatically "ripple through" all the parent drawings to which it's xrefed. This benefit is even greater on larger projects involving multiple drafters, each of whose work may be incorporated in part or in whole in the work of others.

The automatic update feature of xrefs is a big advantage only if you're organized about how you use xrefs. Suppose that an architect creates a plan drawing showing a building's walls and other major features that are common to the architectural, structural, plumbing, and electrical plan drawings. The architect then tells the structural, plumbing, and electrical drafters to xref this background plan into their drawings, so that everyone is working from a consistent and reusable set of common plan elements. If the architect decides to revise the wall locations and updates the xrefed drawing, everyone will see the current wall configuration and be able to change their drawings. But if the architect absentmindedly adds architecture-specific objects, such as toilets and furniture, to the xrefed drawing, or shifts all the objects with respect to 0,0, everyone else will have problems. If different people in your office share xrefs, create a protocol for who is allowed to modify which file when, and what communication needs to take place after a shared xref is modified.

Becoming attached to your xrefs

Attaching an external reference is similar to inserting a block, and almost as easy. Just use the following steps:

1. Set an appropriate layer current.

You should insert xrefs on a separate layer from all other objects. Note that if you freeze the layer an xref is inserted on, the entire xref disappears. (This behavior can be either a handy trick or a nasty surprise.)

2. Choose Insert --> Xref Manager to start the XRef command.

The Xref Manager dialog box appears.

Don't choose Insert --> External Reference. This menu choice jumps ahead to Step 4, which will be confusing at this point.

3. Click the Attach button.

The Select Reference File dialog box appears.

4. Browse to find the file you want to attach, select it, and then click Open.

The External Reference dialog box appears.

5. Specify the parameters for the xref in the dialog box.

Parameters include the insertion point, scaling factors, and rotation angle. You can set these parameters in the dialog box or specify them on-screen, just as you can do when inserting a block.

You can select the Attachment or Overlay radio button to tell AutoCAD how to handle the xref. The choice matters only if you create a drawing that uses xrefs, and then your drawing is in turn used as an xref. Attachment is the default choice, and it means that the xrefed file will always be included with your drawing when someone else uses your drawing as an xref. Overlay, the other choice, means that you see the xrefed drawing, but someone who xrefs your drawing won't see the overlaid file. By choosing Overlay, you can xref in a map, for example, to your drawing of a house, but not have the map show up when someone else xrefs your house drawing. (That person can xref the map, if need be.) Use the default Attachment reference type unless you have a specific reason to do otherwise.

The Path Type drop-down list provides more flexibility in how the xref's path gets stored. For now, choose Relative Path instead of the default Full Path.

6. Click OK.

The externally referenced file appears in your drawing.

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