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Externally Referenced Drawings (Xrefs)
Using blocks to build a convenient component library reduce the drawing overhead associated with repetitive drawing geometry. Now we look at the benefits of using external reference files and discuss appropriate ways of introducing them into your drawing practise.
When a block is inserted in a drawing, a complete and self-contained block definition is embedded in the drawing file. With Xrefs, however, drawing file size does not increase significantly and, more importantly, the referenced geometry is linked. This means that modifications to the Xref file are reflected in all of the drawings it’s attached to. This feature is very useful for assembling master drawings from component drawings — particularly during the design phase of a project when geometry changes impact on other design elements.
The use of Xrefs for drawing borders is a basic but common example. Most drawing offices now Xref drawing borders, providing an easy and convenient mechanism for updating a number of drawings if the company name or address details change. Instead of having to change these details in each and every drawing, only the original drawing needs to be amended — all drawings referencing the original will reflect the latest revision when next loaded. If text within the drawing title block is inserted as an attributed block, we have a good example of Xrefs and blocks combining to take advantage of their relative properties and strengths.
During the design phase of architectural projects, many architects have developed the approach of a common geometry pool, using Xrefs as a publishing tool to produce drawings. With this approach you can develop geometry, such as walls, in drawings that don’t contain drawing borders or title blocks at all. This geometry is then attached as external reference information to a large number of drawing sheets for the creation of working drawings. Attach or Overlay?
When it comes to inserting an Xref (Tools > External Reference), the two options are Overlay and Attach. The Attach option maintains nested Xref files — if I attach an Xref I also attach the files that if references.
If I overlay an Xref that itself contains overlaid Xrefs, the overlays do not appear as part of the external reference. Overlays are used when you want to see reference geometry in a drawing, but you don’t need to include that geometry in drawings that will be used by others.
By using Xrefs during the design stage of a project, members of the design team can work on a common data set and immediately see the impact of changes to shared geometry. The Reload option of the Xref command forces Xrefs to be updated during the drawing session. The Detach option removes overlaid and attached Xrefs.
Now that we’re aware of the power of Xrefs, let’s discuss some of the potential problems. Xref files are linked, rather than embedded, so if referenced files are moved or deleted IntelliCAD provides the message “Unable to locate the externally referenced file”. This also presents a problem if network drive mapping is not consistent across workstations referencing common geometry. Even where IntelliCAD can’t find an Xref, the Xref is still defined in the drawing and can be re-established with the Path option of the Xref command.
This is an important point to note when issuing drawings because it means you must also remember to issue all Xref files. It’s also likely that the Xref paths on your system won’t exactly match the system attempting to view the issued drawings. For this reason it’s good practice to bind or insert Xrefs before issuing or archiving drawings. This process breaks the link and embeds a copy of the Xref file in the current drawing. Insert or Bind?
Both bind and insert options make the Xref a permanent part of the drawing. The main difference between binding and inserting Xrefs is the way drawing layers are handled. If I attach an Xref named NOTE that has a layer TEXT1, IntelliCAD shows me the layer NOTE|TEXT1, which indicates both the Xref file and original layer name. If I bind this Xref, the layer name changes to NOTE$0$TEXT1, which maintains its unique properties and indicates the original layer source. If, instead, I insert the Xref, the layer TEXT1 is created or adopts the properties of an existing TEXT1 layer. At the same time a block definition of NOTE is created within the drawing and can be reinserted.
In summary, try to use blocks for simple library components and Xrefs for sharing geometry. Xrefs provide great flexibility during the design phase but it’s good practice to bind or insert Xrefs before issuing or archiving files.