Tuesday, 12 May 2015

A look at XREFs

 XREF is a great way to quickly and easily connect information from drawing to drawing.  Many users find it daunting, cumbersome, or just simply confusing.  Let's take a closer look.

An XREF (eXternal REFerence) is an external CAD file, image, or PDF that is linked to your current drawing.  AutoCAD loads the file into the current drawing and displays it, and even notifies you if a referenced file has been modified, and prompts you to reload the reference.  You can think of XREFs as blocks from another drawing. 

XREFs have several benefits:

  • Multiple drawings can reference the same file, so updating is easy, and more than one user can access the same data.
  • Layer settings for XREF drawings can be easily manipulated.
  • Since all that is tracked is the insertion, rotation and scale factor, XREFs do not add much to the size of a file.
  • An XREF is like a block, if you freeze the layer it is inserted on, the entire XREF is frozen.
  • When the reference file is modified, all the drawings that have it are updated.
  • 5 types can be XREFed: DWG, DXF, DGN (yes, Microstation files!), PDF (yes, PDF files too!), and image files (JPG, GIF, TIF, PNG, .

Types of XREFs

There are 2 types of XREF: Attachment and Overlay, which you specify in the XREF attach dialog.

Attachment means the file AND all of its XREFs get attached to when XREFed into another drawing.

Overlay means only the selected file gets referenced when XREFed to another drawing; AutoCAD ignores the XREFs that the file may have (no nested references).

To change from one to the other, toggle it in the Details area of the XREF Palette. 

Example: You have a drawing of a site, with a pipeline XREF as Overlay. If the site drawing gets XREFed into another drawing, the pipeline will not display, since overlays do not nest.

Visretain controls the appearance of XREF layers in your  drawing.  If set to 1, any changes to XREF layer properties, visibility, colour, etc is saved in the host file. When visretain is set to 0, all layer settings get reset from the XREF each time the file is loaded or reloaded.

Path and Status
An XREF can have one of several status: loaded, unloaded, orphaned, not found, unreferenced.

Loaded:            The reference is loaded into the current drawing
Unloaded:         The reference is not loaded, but AutoCAD still has the info, and can easily reload it.
Orphaned:         The reference is nested in another reference that has been unloaded. Orphaned references do not display.
Not Found:         AutoCAD cannot locate the reference file.
Unreferenced:   The XREF has been erased from the drawing file, but not detached as an XREF. AutoCAD knows where to find the XREF, but not where it is inserted.


The XREF path can be set as either relative, full, or none.  AutoCAD saves either the full path (drive name, directory structure, file name), relative path (the location of the XREF relative to the host drawing, file name), or none (just the file name). 

TIP#1:   If you want to know what layer, linetype, etc, an object on an XREF is, use the command XLIST: it returns object information without opening the XREF file!

TIP #2:  NCOPY allows copying of selected objects in an XREF directly into your drawing!