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AEC Newsletter archive:
Geoffrey Moore Langdon
CADENCE AEC Tech News # 42 (January 15, 2001)
AEC Tech News 1/15/01
In This Issue:
Image Overlay Drafting
In AEC there are many times when we just need to get something already drawn on paper into CADD - a plot plan from the old building department records, contour lines of a site, a company's logo, or old building plans.
For that last item, plans, and many other situations which require great accuracy, the best option, surprisingly still, is to draft it line by line into CADD from the drawing dimensions. Old drawings/prints/xeroxes are frequently stretched or crumpled, so that even if some automatic scanning system worked perfectly, it would still need a good deal of human interaction to correct the results.
If a project or site is already existing, you may prefer to use one of the site measurement techniques (such as the Leica Cyrax laser system mentioned in AEC Tech News # 27) particularly since things may indeed have changed since that paper drawing you have was done.
There are a number of scanning services who can take 24"x36" and larger sheets and scan them into an image file (tif, bmp, pict, gif, jpg). Personally, I have always taken the cheap trick of xerox-reducing it until it fit in my desktop scanner. Evan Shu, AIA, launched his CheapTricks newsletter 12 years ago with the trick of scotch-taping a clear xerox on the monitor face to trace logos, and has covered dozens more tricks on this subject - such as "faxing the drawing to your fax-modem" - over the years.
However, wherever you got the drawing - a scan, a digital photo, a fax file, a web site, or even terraserver.com - you still have only a digital image file made up of dots or rasters, not lines or vectors as used by CADD.
At this point you have three choices :
1. Leave the drawing as it is in raster format
(There are wonderful things you can do in PhotoShop or CAD OverlayESP, where you can erase portions and add your work, all without bothering to enter the vectorgraphics world of CADD).
2. Use an automatic "vectorization" program such as Adobe Streamline, Scan2CAD, or Corel Trace
(All the lines of the drawing will come up in one color on one layer, consisting of thousands of short bits of lines, and even paper creases, smudges, and text will show up. With smarter programs you Might possibly get choices to trace down both sides of walls that have been filled in, or maybe a real arc instead of thousands of lines designating a door swing, and some will interpret text as text with OCR intelligence. Still, you frequently end up having to trace over these kinds of results to get accurate dimensions.)
3. Use an overlay drafting technique with the image in the background
(Some CADD programs allow you to import a raster image as a background image, allowing you to trace the lines yourself on top. Not only is this technique many times faster than drawing by glancing at side-table references or using digitizers but also you can place appropriate things on the right layers as you trace them, and use your CADD drawing aids such as ortho, snap grids and object snaps.)
You could also go to a "vectorizing service" which will scan your drawings and hand you back nice CADD files with layers. Actually, virtually all of the services make use of that image overlay drafting technique and do it line by line into CADD by hand. In the early 1980's some really bright guys at the School of Architecture at RPI in New York discovered on internship how they could trick AutoCAD into displaying an image in the background with the then new ADI driver. They went on to perfect, refine, and improve what became CAD Overlay which became the standard in a whole new industry. For years this trick was unique to AutoCAD, and people found it was definitly the fastest, easiest, and most accurate way to get existing drawings into CADD. Autodesk bought their company and incorporated the CAD Overlay features into the full versions of AutoCAD 14 and 2000.
Now, there are a host of CADD programs with image overlay features, including
AllPlan, Arris, VectorWorks, ArchiCAD, DataCAD Plus, Bricsnet IntelliCAD, MicroStation, AutoCAD 2000i, and many others.
A key to the workability of this feature, though, is whether (and how easily) you can set the scale of the image before you start tracing. Many programs can place an image simply in the background, allowing you to trace, but then have to use the enlarge/scale command on the resulting lines to blow it up to the appropriate size. Control over the image beforehand, makes it easier to doublecheck dimensions, and use appropriate snap grids, which speeds things up considerably.
This is why, for many years, whenever I was doing all this in an office without access to CADoverlay (which was an over $1000 add on to AutoCAD), I would use ArchiCAD, which also benefited from the better image handling abilities of the Macintosh. The image overlay abilities of several programs has improved significantly in the past two years, notably VectorWorks, ArchiCAD, and AutoCAD.
With AutoCAD2000i and ArchitecturalDesktop2 as well as 3, Autodesk has made the image overlay drafting technique a real pleasure. With a simple right click, you have access to easily change the image scale, rotatation, layer, and more. The import function has also been enhanced so that it easily brings in some 14 different image file types - including Macintosh pict, and even the three tempermental TIFF (scanner) formats. You can now pan and zoom the image as you are tracing, and you can swich tracing layers with a single click. There is even an Image Manager similar to the external reference files manager so that you can easily keep track of many images inserted into the same drawing file. Another feature many of the others do not have, is that you can flip to any elevation view and insert more images in that orientation - making it easy to develop 3D models. There is even a "transparency" toggle, which allows you to combine color rendered images and vector CADD lines which opens a whole world of photocomposite design possibilities. Thus AutoCAD 2000i earns top marks and even ease of use points in an area that is frequently overlooked, but is one of the best reasons to upgrade in my opinion.
In most cases, you are best off to trace with straight polyline segments, even if you are tracing curved contours. This makes it easiest to select, easiest to extrude into 3D later if that is what may be needed, and easiest to transfer to other CADD software. If you are tracing over a photograph of, say, a building entrance, look for the imageclip command, which will allow you to crop the photo to just what you need.
For those who use other CADD software and want to add this ability to their office (and cannot wait for DataCAD 10) or who are on a budget, the least expensive way to get this ability is to get the Bricsnet version of IntelliCAD for $145.
Links :
CheapTricks newsletter = http://www.world.std.com/~eshu/cheap.htm
terraserver.com = http://www.terraserver.com
Adobe Streamline = http://www.adobe.com/products/streamline/main.html
Scan2CAD shareware = http://www.softseek.com
Corel Trace (part of Corel Draw 10) = http://www.corel.com
School of Architecture at RPI = http://www.rpi.edu
AutoCAD 2000i = http://www.autodesk.com
ArchiCAD = http://www.graphisoft.com
AllPlan = http://www.nemetschek.com
Arris = http://www.arriscad.com
Bricsnet IntelliCAD = http://www.bricsnet.com
MicroStation = http://www.bentley.com
DataCAD Plus = http://www.datacad.com
VectorWorks = http://www.vectorworksarchitect.com
Cinema 4D = http://www.maxon.de
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About Geoffrey Moore Langdon, AIA
Prof. Langdon is a registered architect and is the principal of Architectural CADD Consultants, a firm that specializes in helping architectural firms with computing and CADD. He has taught Design, Solar Energy, and Architectural CADD at a number of colleges in the Boston area. He is the author of Architectural CADD: A Resource Guide to Design and Production Software Appropriate for Architects, a guest speaker at many AIA events, and the founder and organizer of the Designers 3D CAD Shootout competition. contact him at aectechnews@architecturalcadd.com, or through his website: http://www.architecturalcadd.com
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