Optimizing Layers in CAD Drafting

As an architect in Cincinnati, Ohio, I do a lot of computer-aided-drafting (CAD), and I receive CAD files from other firms as well. I notice a striking difference between the files that I produce and the ones created by others, specifically in the profusion of layers established within the file. This is likely due to a difference in how our respective CAD software functions, but may also simply harken back to limitations imposed in the early days of CAD that have now become engrained work habits.

I use drafting software called Graphite, by Ashlar, which has the feature that line weight, style, and color are independent of layer assignment. In older versions of AutoCad, which most others use, line weight, style, and color were determined by layer. In part, this was tied to how the CAD drawings were printed, using a plotter that selected among several actual pens stored in its carousel to render a particular item; this selection was laxer-based.

A good architectural drawing is one that communicates, and good architects know that employing a variety of line weights and styles can aid the human eye in discerning the meaning of the drawn elements. A heavy solid outline can, for instance, define the overall mass of an object, while lighter lines describe its surface features (like the lines of siding and trim on an exterior wall elevation). In my firm, we go one step further, because the large format printers of today are equally adept at printing in full color: we use color as a graphic aid. As an example, we show existing conditions in green, so that new work shows up bold against it in black. We use blue for dimensions so that their witness lines do not get confused for building elements.

Since we have the ability to define all three characteristics (weight, style, and color) independent of layer assignment, we prefer to optimize the number of layers used. Of course, one can just draft every element within a single layer, but this sacrifices the control offered by an intelligent layer structuring.

My thinking about layers developed while I was first working as a draftsman, in firms that had yet to adopt CAD and instead were using some form of manual drafting. I became familiar with the concept of the pinbar, which was a strip of metal with knobs at regular spacing, which was fastened to the drafting table. Each sheet of mylar then had corresponding holes punched along its top edge. This allowed one to situate one sheet on top of another, with perfect alignment... in other words, create a layer. When it came time to print, clear plastic buttons could hold the sheets together during blueprinting. This method had one major limitation, though, in that you could not stack more than about four sheets for any one drawing. If you did, the bottom sheet would be too obscured, and would print faint and diffuse.

Thus, one had to consider what sorts of information was going to be needed on each sheet. A base plan might serve for both the main floor plan and for the reflected ceiling plan, as well as possibly for a furnishings plan, etc. Information that was only going to appear on the reflected ceiling plan needed to go on the RCP mylar. Information that would appear on both the floor plan and the RCP but not on a furnishings plan, though, would need its own mylar layer. The decision to go from 2 to 3 layers, though, was never taken lightly, since there is a cost in terms of blueprint clarity with each additional layer.

In the modern paradigm, that blueprint clarity criteria no longer applies, but in my opinion a clarity of another sort does matter. I refer to the clarity of the CAD operator's understanding of the file's layer structure. The fewer layers there are, the better the draftsman can understand how they should be used. Having made that statement, I know that I do see a lot of CAD files in which a separate layer has been assigned for almost every distinct element, sometimes resulting in hundreds of layer names. While that approach does allow for clarity regarding which layer contains which elements (i.e. the "Refrig" layer contains the refrigerator), what it lacks is any intuitive way for a draftsman to know which layers to turn on or off in order to view the file as intended. It forces any draftsman who is given the file to go sift through the long list of layers until they find what they are looking for.

In my approach, I try to think through how many different ways a given CAD file will need to present itself, and structure layers with the minimum number to achieve that. As described above, a floor plan might serve as a base drawing for a reflected ceiling plan, a furnishings plan, even a demolition plan, so I build in unique layers that allow those conversions. But there is also the notion of how the file presents itself to the draftsman, meaning that sometimes it can be good to segregate items onto a layer even if that layer is never toggled off when printed. As an example, on a Site Plan I prefer to have all contour lines on a "Topography" layer, so that I can turn it off while I am working with laying out a parking lot or other features. That way, I won't be accidentally snapping lines to a contour line or finding X/Y alignments to the myriad vertices in a typical contour line; it speeds up my productivity.

Since I can use color freely, too, I can let one layer serve many purposes. On a "Notes" layer, I might have regular text notes in black with leader arrows pointing to the things they describe, but I could also have all the door numbers in Turquoise and all the partition type flags in orange. The color helps viewers understand the meaning of the tags or symbols, but since they all print together they all go on the same layer. A "Demolition" layer can contain all the dotted or dashed lines, text notes and leader arrows, and whatever else goes into converting the base plan into the Demo Plan.

Sometimes I find myself economizing the layers after the fact, which may be a worthwhile way to work. The file might have been created with a number of layers which really don't need to be separated, and so I might grab everything on one of those layers and move it to a different common layer. When I do, though, I usually assign those elements a unique color, so that I still have something to distinguish them later on.

One final use of layers has to be mentioned, and that is for non-printing features like alignment lines or referential elements that are not part of the actual drawing. As an example, I often create an "Area" layer on which I can trace out the floor plan in order to calculate its square footage. When I am done with those calculations, I can just turn the layer off, and if I ever make plan adjustments I can turn it back on and recalculate. Another example would be the development of an exterior elevation by means of drafting a quick study of a building or wall section; this would be very rudimentary and just enough to inform the conditions shown on the elevation drawing, almost like an X-ray showing the building's "bones".

In the end, my drawings do end up with a good number of layers, that each serve very specific purposes. I do not, however, produce dozens or hundreds of layers that befuddle an outsider.

Article Source: Michael Rountree

1 comment:

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