The first step is to attach the external reference, which is just another drawing, to your current (host) drawing. You can underlay DWF and DWFx files, which are covered in Chapter 28, DGN and PDF files. I explain underlays at the end of this chapter.

To attach an xref of an AutoCAD or AutoCAD LT drawing, follow these steps:

If your current view does not show the entire xref, do a ZOOM Extents (choose View tab

After you have the xref in your drawing, you can start to work. The xref is like a block, although you cannot explode it. However, you can use object snaps on all the objects in an xref, just as you can with blocks. This enables you to use the xref as a basis for your own drawing.

Sometimes you need to open the xref to work on it directly. For example, you may see an error that you want to correct. The XOPEN command opens xrefs. The easiest way to use XOPEN is to click the xref to select it in your drawing, right-click, and choose Open Xref. The xref opens in its own drawing window.

To see what type of xrefs you have in your drawing, choose View tab

Figure 19.2. The External References palette displays referenced drawings, raster images, DWF and DWFx files, DGN drawings, and PDF files.

The palette lists all the referenced drawings and files. You can choose one of two views, using the buttons at the top-right corner of the palette:

When you click any xref, you see a preview tooltip that shows the reference and lists its details. You can control the preview format to display a preview thumbnail, file details, or a combination. Right-click in the palette, choose Tooltip Style, and choose one of the options.

If you move or delete a referenced item, the External References palette notes that the file is not found. To troubleshoot, it helps to know where it searches for xrefs. The External References palette searches for references according to a specific order:

STEPS: Attaching Xrefs

As you click each of the available references, its preview appears at the right. If the xref has nested objects, choose one of the following options (if it does not, then ignore this section of the dialog box):

Figure 19.4. The Reference Edit dialog box enables you to choose which reference you want to edit, including nested references.

For more control, click the Settings tab to set the following options:

Click OK to close the Reference Edit dialog box.

If you checked the Prompt to Select Nested Objects option, you see a prompt to select nested objects. Complete object selection to define the working set, that is, the objects that you can edit. Other objects are faded by 50 percent (this is the default, determined by the XFADECTL system variable).

The Edit Reference panel appears on the current tab, shown in Figure 19.5, and you see the message Use REFCLOSE or the Refedit toolbar to end reference editing session on the command line. You're now ready to edit the xref or block.

Figure 19.5. The Edit Reference panel on the Insert tab.

There are several types of edits that you can make on the working set of objects from the xref or block:

When you save changes to a block, block definitions are redefined and all instances of the block are regenerated according to the new definition. If you gave an xref object properties that don't exist in the xref, such as a layer, the new property is copied to the xref so that the object can keep that property.

STEPS: Editing an Xref in Place

Dependent symbols are named items in a drawing, such as layers, text styles, dimension styles, and so on. When you attach an xref, these symbols are listed in your current drawing. For example, the Layer Control drop-down list displays the layers of the xref. Xref symbols have the format xref_name|symbol_name. This system distinguishes xref symbols from those of your current drawing and ensures that there are no duplicate symbols.

The XBIND command

You can use the XBIND command to import only the symbols that you want from the external reference into the current drawing. This makes it easy to work with a consistent set of symbols in the current drawing and the xrefs. For example, you can choose to import the titleblk layer and the dec dimension style. On the command line, type xbind

Figure 19.7. Using the XBIND command to import symbols, such as layers, text styles, and so on.

Click the plus sign next to any symbol type to open a list of symbols. Click the one you want and choose Add to add it to the Definitions to Bind list. Click OK when you're done.

Later in this chapter, I explain how you can use the DesignCenter to move xrefs and other dependent symbols from one drawing to another.

If drawing a includes drawing b as an xref and drawing b includes drawing a as an xref, then you have a circular reference. Circular references can exist among three or more xrefs when you have nested xrefs. The program detects circular references and loads as much as it can. If you try to load an xref in such a situation, you see the following message:

Warning: Circular reference from XREF to current drawing.
Circular reference(s) have been found. Continue? <N> Type y to continue to load the xref.
Breaking circular reference from XREF to current drawing.

You may want to see only part of an xref. This option is especially important when you're using very large xref drawings. The CLIP and XCLIP commands enable you to create a border in an xref, and hides any part of the xref outside the border.

Table 19.1 explains the options of this command.

To see the clipping boundary (if you haven't used an existing polyline to define it), change the value of the XCLIPFRAME system variable to 1. When you do so, you can select the boundary and modify it by using its grips. You can also click the blue arrow to invert the clip.

Figure 19.8 shows an xref clipped with a polygonal boundary.

In order to reduce the time needed to display large xrefs, such as those used in GIS or 3D drawings, you can use demand loading. This feature enables you to load only the objects necessary to display the xref in your drawing. Demand loading works together with spatial and layer indexes.

Figure 19.8. An xref clipped with a polygonal boundary.

To make it perfectly clear, you need all the following in order to use demand loading:

Demand loading

You can turn on demand loading in your current drawing. To turn on demand loading, choose Applications Button

Spatial indexes

You save a spatial index for a drawing that you expect to use as an xref. The saving process takes a little longer, but you save time at the other end when you load a clipped xref or clip an xref for the first time. To create a spatial index in AutoCAD, choose Application Button

From the Index Type drop-down list, choose Spatial or Layer & Spatial. Click OK. Then click Save in the Save Drawing As dialog box. In AutoCAD LT, use the INDEXCTL system variable on the command line and set its value to 2 for just a spatial index, or to 3 for both spatial and layer indexes.

Tip

If you want to create an index for an existing drawing, click OK once to return to the Save Drawing As dialog box. Click Cancel. In other words, you don't have to actually save the drawing to set up the index, which is controlled by the INDEXCTL system variable.

After you create a spatial index, each time you save the drawing, you see the following message:

Updating Indexes for block *MODEL_SPACE

Figure 19.9. The DWG Options tab of the Saveas Options dialog box enables you to save spatial and layer indexes.

To stop saving the index each time you save in AutoCAD, choose Application Button

Layer indexes

You save a layer index for a drawing that you expect to use as an xref to create an index of all the layers in the drawing. As with a spatial index, the saving process takes a little longer, but you save time at the other end when you load an xref with frozen or turned off layers. To create a layer index in AutoCAD, choose Application Button

From the Index Type drop-down list, choose Layer or Layer & Spatial. Click OK, and then click Cancel. In AutoCAD LT, set the INDEXCTL system variable to 1 for just a layer index, or to 3 for both layer and spatial indexes.

After you create a layer index, each time you save the drawing, you see the following message:

Updating Indexes for block *MODEL_SPACE

To stop saving the index in AutoCAD, choose Application Button

Note

The drawings used in the following exercise on controlling xref display — ab19-a.dwg, ab19-b.dwg, ab19-01.dwg, and ab19-02.dwg — are in the Drawings and Results folders on the DVD.

STEPS: Controlling Xref Display

1. Open ab19-01.dwg from your AutoCAD Bible folder if you did the first exercise in this chapter.

   If you didn't do the first exercise in this chapter, use Windows Explorer to find ab19-b.dwg in the Drawings folder on the DVD and ab19-01.dwg and ab19-02.dwg in the Results folder on the DVD. Copy all three files to your AutoCAD Bible folder. In Windows Explorer, right-click each file and choose Properties. Uncheck the Read-Only option (if checked) and click OK. Then open ab19-01.dwg from your AutoCAD Bible folder.

   If you get a message that the drawing cannot find the xrefs and you are using AutoCAD, you can choose Application Button

   

2. To create layer and spatial indexes, do one of the following:

   • If you have AutoCAD. Choose Application Button

     

   • If you have AutoCAD LT. On the command line, enter indexctl

     

3. Choose Save from the Quick Access toolbar. Note the message on the command line that the indexes are being updated.

4. Close ab19-01.dwg.

5. Open ab19-02.dwg from your AutoCAD Bible folder. This drawing has an attached xref of a house plan and a nested xref of a titleblock, as shown in Figure 19.10.

6. Save it as ab19-05.dwg in your AutoCAD Bible folder.

7. Choose Application Button

   

8. Click the Layer Control drop-down list. Click the On/Off icon next to the Ab19-01|notes layer to turn the layer back on. Click the top of the drop-down list box to close it. The notes layer displays.

   

   Figure 19.10. Picking a polygonal boundary to clip an xref.

9. 

   Select objects: Pick anywhere on the xref in Figure 19.10.
   Select objects: 
   
   Enter clipping option
   [ON/OFF/Clipdepth/Delete/generate Polyline/New boundary] <New>: 
   
   Specify clipping boundary:
   [Select polyline/Polygonal/Rectangular/Invert clip] <Rectangular>: Right-click and choose Polygonal.
   Specify first point: Pick 
    in Figure 19.10. (It might help to turn off Object Snap if it is on.)
   Specify next point or [Undo]: Pick 
   .
   Specify next point or [Undo]: Pick 
   .
   Specify next point or [Undo]: Pick 
   .
   Specify next point or [Undo]: Pick 
   .
   Specify next point or [Undo]: Pick 
   .
   Specify next point or [Undo]: Pick 
   .
   Specify next point or [Undo]: Pick 
   .
   Specify next point or [Undo]: Pick 
   .
   Specify next point or [Undo]: Pick 
   .
   Specify next point or [Undo]: 
   

   This action clips the xref.

10. Turn off the Ab19-01|notes layer again.

11. Save your drawing. It should look like Figure 19.11. Keep the drawing open if you're continuing on to the next exercise.

    

    Figure 19.11. The clipped xref.

If an xref is moved or renamed while you have it displayed in an open drawing, you need to reload it. An xref can change if someone else on your network opens and edits it while you're using it. External Reference Notification offers instant notification if an xref changes.

As explained in earlier chapters, you can use the DesignCenter to move named objects, including xrefs, from one drawing to another. (I cover the DesignCenter in detail in Chapter 26.) To insert an xref from another drawing, press Ctrl+2. Navigate to the drawing and double-click it to open the list of named objects. Double-click Xrefs to see a list of xrefs in the right pane.

Double-click the xref that you want to insert. The Attach External Reference dialog box opens (refer to Figure 19.1) so that you can insert the xref. You can also right-click and choose Attach as Xref, or right-click and drag a drawing from the DesignCenter into your drawing.

If you set the XREFCTL system variable to 1 (by default, it's set to 0), a copy of all xref activity for your current drawing is saved in an ASCII text file. You can read the log to troubleshoot problems that may occur. The log file goes in the same folder as your drawing and uses your drawing name with the .xlg filename extension. This file can become long. Therefore, once in a while, you should delete all or part of the file.

STEPS: Managing Xrefs

Attaching a DWF file is similar to attaching an xref or inserting a block. To attach a DWF file, follow these steps:

If your current view does not show the entire underlay, use ZOOM Extents to see it (choose View tab

If the drawing has more than one model, choose the one you want to attach from the Select a Design Model from the DGN File list box. DGN files can have master units (Imperial or metric) and another set of units, called sub units. Choose which one you want to use. AutoCAD converts one unit (either master or sub) to one drawing unit. Click OK to attach the DGN underlay.

After an underlay is created, you can control its display and how you can work with the underlay's geometry.

Clipping an underlay

If the underlay is large, you may not want to view all of it in your drawing. You can clip an underlay in the same way that you can clip an xref or block, by using the CLIP command. To access this command, choose Insert tab

• ON. Turns on an existing clipping boundary. By default, a clipping boundary is on.

• OFF. Turns off an existing clipping boundary so that you can see the entire underlay.

• Delete. Deletes a clipping boundary.

• New boundary. Creates a new rectangular or polygonal clipping boundary.

Note

The CLIP command allows you to create a clipping boundary for any underlay from a single command. You can also use the DGNCLIP, DWFCLIP, and PDFCLIP commands, based on the type of underlay you are working with.

Displaying the frame of an underlay

You can turn the display of the frame for an underlay on or off. This is very similar to the frame that is displayed after clipping an xref or block. If the underlay is not clipped, the frame shows the rectangular border of the entire underlay. If you clipped the underlay, the frame shows the clipping border.

To display the frames for all underlays in a drawing, use the FRAME system variable and set its value to 1 (displays the frame and plots it) or 2 (displays the frame, but doesn't plot it). To turn off the display of all frames, set the value to 0. Even with the frames turned off, you can select an underlay. To change the display of the frames for underlays, choose Insert tab

Note

The FRAME system variable allows you to control the display of the frames for all xrefs, underlays, and raster images attached to a drawing, regardless of the settings of the other frame-related system variables.

When FRAME is set to 3, you can control the frame display for xrefs, underlay's, and raster images in the drawing by using the appropriate system variable. Use the following system variables to control the display of the frames for underlays; except for value 3, they support the same values as FRAME:

• DWFFRAME. Controls the frame display for DWF underlays.

• DGNFRAME. Controls the frame display for DGN underlays.

• PDFFRAME. Controls the frame display for PDF underlays.

Using object snaps with underlays

By default, you can use object snaps with underlays. If you need to use the object snaps of your underlay, this is the ideal setting. In PDF underlays, object snaps only work with vector-based objects; they do not work with raster-based (bitmap) objects.

However, if you don't want to use object snaps with underlays or you find yourself getting "caught" on an underlay's object snaps, you can turn object snaps off. To change the status of object snaps for underlays, choose Insert tab

Note

The UOSNAP system variable is new for AutoCAD 2010 and AutoCAD LT 2010.

You can control the use of object snaps by underlay type, using the following system variables:

• DWFOSNAP. Controls the use of object snapping with DWF underlays.

• DGNOSNAP. Controls the use of object snapping with DGN underlays.

• PDFOSNAP. Controls the use of object snapping with PDF underlays.

Note

When you set one of the underlay-specific system variables that control object snap settings to a value that is not equal to the current value of UOSNAP, UOSNAP changes to a value of 2. This value honors the underlay-specific snap settings for underlays.

Control the layers of an underlay

You can control the layers that are displayed for an underlay if the attached file was published with layer information. When you publish a DWF or DWFx file, you use the Publish Options dialog box to set Layer information to Include. (See Chapter 28 for more information on DWF and DWFx files.) The layers in a DGN underlay are based on the levels used in the original DGN file. PDF files can contain layer information if the application that created the PDF file saved the information during output.

Note

When you plot or publish to a PDF file, AutoCAD 2010 or AutoCAD LT 2010 saves layer information.

Use the ULAYERS command to turn layers on or off for an underlay. To access this command, choose Insert tab

Note

The ULAYERS command allows you to control the display of the layers for any underlay from a single command. You can also use the DGNLAYERS, DWFLAYERS, and PDFLAYERS commands for the specific types of underlays.