A Brief Overview of AutoCAD: From Xrefs to Viewports
Let us first begin by defining XREFs and how they are important in architectural working drawings. To be brief, XREFs are Cross-Referenced drawings. What this means is that they are independent drawings which are referenced into other drawings. In other words, they are in effect “attached” to the drawing. You can also “overlay” an XREF (The difference between attaching an XREF and overlaying an XREF is beyond the scope of this article.) In order to understand this concept more clearly, let me show an example:
A Drafter named Michael has been assigned the task of a new project. He has gone out to the job site and done the field investigation. His firm is taking an existing building’s shell and remodeling it into a retail space. The drafter has taken the day to measure and photograph as many aspects of the job site as he can. To double check himself he has made sketches in his notebook and he then takes all the information he has gathered and is ready to transfer that information into AutoCAD. Where to begin…?
Model Space, Paper Space and Viewports
Michael has decided he wants to start by drafting everything in Model Space. This is an excellent idea. He has everything he needs to get started and uses AutoCAD to draft his floor plan. But what is the difference between Model Space, Paper Space and what is this thing called a Viewport? I am glad you asked. Some people get confused between Model Space and Paper Space and Viewports. I’ll let you in on a secret. It’s easier than you might think! Here are the definitions of these features of AutoCAD:
Model Space: On a clean black canvas of AutoCAD drafting space, Model space is that area of the AutoCAD universe where everything is drawn to scale. To say this another way, if a wall is measured to be 11’-6” (3.5052 m) then it is drawn at 11’-6” (3.5052 m) in Model Space. Think of this space as an area where you can “model” what you are building.
Paper Space: Now this might be a bit easier to understand. In Paper Space you are dealing with a sheet of paper. Whatever the size of it. Usually for an architectural drawing this is a drawing 30”x 42” (0.762 m x 1.0668 m) for a full-size sheet. (It may vary depending on size of project and the firm involved). In Paper Space everything drafted is at a scale of 1:1. This is usually where the Title Block is attached (as an XREF (Cross Referenced Drawing)) in Paper Space.
Viewport: A viewport is in its most fundamental terms a window. A Viewport is a window cut in the fabric of Paper Space. Or, if you will, a “hole” cut in your paper looking into Model Space. Each Viewport can be scaled relative to Paper Space. One can have many viewports on one sheet of paper, each viewport with a different scale.
So now that we know some basics, let’s catch up with our friend Michael.
Michael has drafted his floor plan in Model Space and has named it in accordance with his company’s policy. Now he is ready to create a new sheet. Michael so far has been the only one working on this file, but now his Engineers contact him and ask him for the file. He emails it over to them so that they can plan their layouts. If any major changes are made, of course Michael will have to send over a revised floor plan to them so that they can have the “latest and greatest” at all times.
One of the main benefits of XREFs is that when they are "XREFed" into (attached to) multiple drawings, and changes are made, they only need to be made once in the original drawing and not in every single file.
So Michael is now ready to create his Floor Plan Sheet. He has created his title block and attached it in PS (Paper Space) at the coordinates 0,0. (The lower left-hand corner of the sheet). He enters Model Space in the new sheet and attaches his floor plan at 0,0 as well. Usually a convenient corner of the building is chosen such as the intersection of the South and West Walls for the insertion point . Michael now goes back to paper space and creates his viewport in the title block. He makes the scale ¼” = 1’-0” in his viewport and adjusts the drawing until it fits within the viewport completely. Michael chooses to enter all his text in PS as well as his notes and North Arrow and Titles.
The Importance of Xrefs
Once understood, one can very easily see how XREFs are extremely convenient. Some of their advantages are that they do not have to be scaled. They can be drafted full size and then scaled in the paper space sheet. Changes do not have be made multiple times, they can be made once and everywhere they are cross referenced, the changes will appear. Floor Plan XREFs can be used for Plan Details and enhanced to show exactly how a building (or portion of a building) is constructed. Engineers can utilize architectural cross-referenced drawings (XREFs) to save time on their projects as well. Everybody is on the same page and time and money are saved in the long run.
Michael finishes this part of his set of construction documents and begins with the elevations. But that is a story for another day.