Xref Clipping of Building Plans

by Mark Middlebrook

A common CAD challenge is how to break up a large model or drawing into multiple plot sheets. With building plans, this problem results from the desire to show a plan at a readable scale (e.g., 1/8"=1’-0") while presenting it on a sheet of paper of reasonable size (e.g., 30"x42"). These two requirements obviously limit the building area that you can show on one sheet. As a result, drafters usually "carve up" larger building plans onto multiple plot sheets, with match lines to indicate the imaginary seams between the plan areas.

Before R14, there were two common ways to organize building plans with multiple sheets and match lines in AutoCAD:

• Simply draw each plan portion in its own DWG file in model space.
• Draw the full plan in one DWG file, xref it into multiple "plot sheet" DWG files, and in each plot sheet create a paper space viewport displaying only the desired area.

The first method is simple to set up, but often becomes awkward as you develop the drawings. You have to jump among several drawings in order to make changes in the vicinities of the match lines. Even worse, if, as is common, you show a small overlapping area beyond the match line on each sheet, then you have to maintain duplicate information. The result is extra editing work and an increased likelihood of inconsistencies in the drawings.

The second method avoids these problems by keeping the entire plan together in one DWG file. But the paper space method does involve extra drawing setup effort, including the creation and scaling of viewports.

AutoCAD R14 and 2000 present a third possibility:

• Draw the full plan in one DWG file, xref it into multiple "plot sheet" DWG files, and in each plot sheet clip the xref so that only the desired area displays.

This new third method is similar to the older paper space approach, but it isn’t limited to paper space or rectangular viewports. There’s nothing wrong with using paper space, but the xref clipping procedure is more intuitive than creating paper space viewports and zooming the model inside of them. With xref clipping, you can create everything on your plot sheet – clipped plan, title block, and additional notes – in model space if you want to. Also, you can clip xrefs to irregular polygonal areas, which avoids the rectangular limitation of paper space viewports.

Example Step-by-Step Procedure

1. Download and open the sample drawing xref_clip_plan.dwg. It is set up for 1/8"=1’-0" (i.e., 1=96) and limits of 0,0 to 360’,120’ (a bit larger than the plan area). The plan is drawn in model space.
2. Note that there are two rectangle enclosing the two areas to be shown on the two plot sheets> These are draw on a layer called S-anno-clip. The areas overlap slightly and contain a match line in the overlap area, as shown in figure 1.



3. Create a new drawing called PlotSht1.dwg. Set it up for 1"=1" (full scale) and limits of 0,0 to 36,24. Create a title block with XREF, INSERT, and/or drawing commands.
4. Use XREF Attach to attach Plan.dwg at a scale factor of 1/96 (i.e., the inverse of the plan’s drawing scale factor).
5. Use XCLIP to clip the xref to the first polyline boundary:
   Command: XCLIP
   Select objects: select the plan xref
   Select objects: ENTER
   ON/OFF/Clipdepth/Delete/generate Polyline/<New boundary>: ENTER
   Specify clipping boundary:
   Select polyline/Polygonal/<Rectangular>: ENTER
   First corner: snap to one corner of the clipping polyline
   Other corner: snap to the opposite corner of the clipping polyline



6. Move the clipped xref so that it’s properly situated inside the title block. Draw a break line at the "cut" edge of the plan. Figure 2 shows the result (with the Plan|X-anno-clip layer turned off).
7. Save PlotSht1.dwg, then use SAVEAS to create a new copy of it called PlotSht2.dwg.
8. Use XCLIP Delete to remove the clipping boundary.
9. Use XCLIP to clip the xref to the second polyline boundary (zoom out as required to snap to the polyline rectangle’s corners).
10. Move the clipped xref so that it’s properly situated inside the title block. Move the break line to the "cut" edge of the plan. Figure 3 shows the result.



11. Before you plot PlotSht1.dwg and PlotSht2.dwg, turn off the X-anno-clip layer. (If you leave VISRETAIN=1 in the two plot sheet drawings, turn off Plan|X-anno-clip in PlotSht1.dwg and PlotSht2.dwg. If you set VISRETAIN=0 in the two plot sheet drawings, temporarily turn off X-anno-clip in Plan.dwg).

Always make editing changes to the plan in Plan.dwg. PlotSht1.dwg and PlotSht2.dwg are for plotting only.

Tips

• Although you can use XCLIP to clip xrefs to non-rectangular boundaries (e.g., an L-shaped area), the clipped area must be polygonal (i.e., composed of straight sides). If you use a polyline with arc segments for clipping, AutoCAD treats the arc segments as straight for the purposes of performing the clipping.
• If you save your R14 drawing to the R12 DWG format, the clipping information will be lost. If you save the R14 drawing to the R13 DWG format, the clipping information will be retained, but the xref will appear unclipped in R13. Thus xref clipping isn’t a good method if you have to share drawings with users of previous AutoCAD versions.
• Paper space viewports retain one advantage over clipped xrefs: the ability to freeze layers in viewports. This feature doesn’t make much difference if you create only one viewport or clipped area per plot sheet, because you can turn on VISRETAIN in the plot sheet drawings in order to save xref layer settings individually in each drawing. But if you show more than one viewport or clipped area in a single plot sheet drawing, paper space viewports afford separate layer visibility control in each one.
• AutoCAD LT does not include the XCLIP command. AutoCAD LT will display and plot drawings with clipped xrefs properly, but LT cannot you cannot create or manipulate clipping boundaries with LT.)

Return to the markcad.com series of AutoCAD tutorials on xrefs.