LT On-line: Lesson 9

Xrefs: blocks writ large

Page 1: Introduction to Xrefs

My previous three LT Online lessons covered the ins and outs of using blocks. After you've mastered blocks, you can graduate to xrefs, which extend the block idea to hierarchies of multiple files. The concepts and procedures described here work with AutoCAD LT 98 and 2000 and AutoCAD Release 14 and 2000.

About xrefs
An xref, or external reference, is like an industrial-strength block. An xref points to a separate drawing outside the drawing you're working on. The referenced drawing then appears on screen and on plots as part of the original drawing, but continues to "live" as a separate document on your hard disk. This arrangement lets you include a whole separate drawing without increasing the size of the drawing you're working on. But you can't explode the xref; you can only change its appearance by editing the externally referenced drawing. If you do edit the externally referenced drawing, the appearance of the drawing changes in all the other drawings that reference it, too. As shown in figure 1.

Figure 1.

Creating a live link to an external drawing is attaching an xref. This procedure is analogous to inserting another DWG file into the current drawing as a block. The most important difference is that AutoCAD maintains a live link to the externally referenced drawing.

Drawings that you include as xrefs in other drawings are often referred to as child drawings, while drawings with pointers to the child drawings are parent drawings.

The big advantage of xrefs over blocks is that if you change a child drawing, AutoCAD LT automatically loads the change into all of the parent drawings that reference that child drawing. AutoCAD LT loads all xrefs into the parent drawing each time you open the parent drawing. If you change the child drawing, the parent drawing automatically incorporates those changes.

Have you ever used Windows OLE (Object Linking and Embedding) to link a document from one program to a document in another program? Attaching an xref is similar to linking a document. Fortunately, xref reliability and performance are good (whereas OLE reliability and performance ranges from okay to awful).

Xrefs: blocks writ large
  Page 1: Introduction to Xrefs
  Page 2: Attaching and Layering