Doug Ramstad, a CAD technician and drafter, sent us a tip for managing the display of multiple viewports in AutoCAD Civil 3D.
"I have Civil 3D 2012, but I use the workspace 2D drafting and annotation most of the time. Sometimes I have issues with viewports. I will have several viewports on a sheet (the typical different views), and the model will become invisible in one of the viewports only to reappear later, and then a different viewport model will become invisible. It prints just fine; I just can’t see the view on my screen. My first thought was that I had a corrupted viewport or file. I ran Audit, etc., but with no effect. After searching blogs on the web, I found a system variable that fixed the problem: MAXACTVP. It's set to 4 by default, but I changed it to 64 and the viewports have been working fine ever since."
Notes from Cadalyst Tip Reviewer Brian Benton: The system variable MAXACTVP in AutoCAD (and its verticals, which include Civil 3D) limits the number of active viewports allowed in a paper space tab at any given time. It is a drawing-based variable (meaning it can be different in every file if you so choose) that can be set from 2 to 64. It will not affect the plotting of viewports, just the display of them on your screen. This is done to help reduce screen refresh rates. A drawing tab that has many busy viewports may be reducing your computer’s performance levels; changing this variable to a lower setting can help. Can you imagine having 64 viewports in your drawing tab?