Working With External References

Save yourself a ton of rework time by using Xrefs effectively

Xrefs infographic
James Coppinger

External references — usually abbreviated as Xrefs — are one of the most important concepts to understand in a CAD environment. The idea is simple enough: link one file to another so that any changes made to the source file will show up in the destination file as well.

Xrefs Explained

Imagine that you have a set of 300 drawings and the title block calls out the number of files — e.g., 1 of 300, 2 of 300, etc. If you have put your title block in every plan as simple text then when you add another drawing to your set, you'll need to open every single file and modify the sheet numbers one at a time. Think about that for a moment. You'll need to open a drawing, wait for it to load, zoom to the text you need to change, modify it, zoom back out, then save and close the file. How long does that take, maybe two minutes? Not that big of a deal for one file but if you need to do 300 of them, that's ten hours of time you'll spend just to change one piece of text.

An Xref is a graphic image of an external file that appears, and prints, inside your drawing just as if it were drawn inside that file. If you created a single title block and inserted the "graphic snapshot" of an Xref into each of those 300 plans, all you need do is update the original file then the Xref in the other 299 drawings is immediately updated. That's two minutes versus ten hours of drafting time. That's huge savings.

How Xrefs Actually Work

Beautiful design
small_frog // Getty Images

Every drawing has two spaces that you can work in: model and layout space. Model space is where you draw items at their actual size and coordinate location, while layout space is the place where you size and arrange how your design will appear on a sheet of paper. Whatever you draw in model space of your source file can be referenced into either model or layout space of your destination file, but anything you draw in layout space cannot be referenced into any other file. Put simply: anything you want to reference needs to be created in model space, even if you plan to display it in layout space.

Although specific procedures vary by software program, the general process is straightforward:

  1. Create a new drawing to serve as your source file.

  2. Draw whatever items you want to reference in model space of the new file and save it.

  3. Open any other file to serve as your destination file.

  4. Execute the Xref command and browse to the location where you saved your source file.

  5. Insert the reference at a coordinate location of 0,0,0 — a common point to all files.

Everything you drew in the source now displays in the destination file(s) and any change you make to the source drawing is automatically displayed in every file that references it.

Common Uses of Xrefs

The uses for Xrefs are limited only by your own imagination but each AEC industry has some fairly typical uses for them. For example, in the infrastructure world, it's common to link several drawings together in a linear "chain" so that changes to each level of the chain appear downstream. It's common to reference your existing conditions plan into your site plan so you can draw your proposed site features on top of your surveyed items. Once that's complete, you can reference the site plan into the utility plan so that you can tie your storm sewer to your new design and the existing pipes because the reference will display both plans as part of the chain.

In the architectural field, floor plans are commonly referenced into other plans such as HVAC and reflected ceiling plans, so that any changes made to the floor plan are immediately displayed in those plans, making it easier to adjust designs on the fly. In all industries, title blocks and other common drawing information are regularly drawn separately and referenced into every drawing in the plan set to make for simple, single point modifications to elements common to every plan.

Types of Xrefs

FreeCAD
Courtesy of FreeCAD

There are two distinct methods — attachment and overlay — for inserting references into a destination file:

  • Attachment: An attached reference allows you to nest multiple references together to create a "chain" effect. If you reference a file that has five other files already attached to it, then the contents of all six files will appear in the active drawing. This is an important feature when you're trying to design different systems on top of each other, yet maintain the ability for multiple people to work on different files simultaneously. For example, Tom can work on "Drawing A," Dick on "Drawing B," and Harry on "Drawing C." If each is attached in that order, then Dick can instantly see every change Tom makes, and Harry sees the changes from both Tom and Dick.
  • Overlay: An overlay reference does not chain your files together; it only displays files one level deep. This approach is useful when the source references for each file don't need to be displayed in every file that comes after it. Let's assume that Dick needs to see Tom's work to complete his design, but Harry only cares about what Dick is drawing. In such a case and overlay is the right way to go. When Dick references in Tom's file as an overlay reference, it will display only in that file and is ignored by "upstream" drawings, such as Harry's. Xrefs are a great tool for streamlining CAD work and ensuring consistent design across multiple files.