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Representation In Lines

Continuing yesterday’s topic, here’s an example of the difficulty in extending the standard drafting language used for structural drawings. I’m going to draw on two sources – my possibly inaccurate memory of the standard line types at my first engineering job in the late 1980s, and release 2.1 of the A/E/C Graphics Standard of the US Army Corps of Engineers, from August 2019.

When you draw a given line (by hand or in CAD) it has three attributes that can vary that are independent of the line’s length and curve: color, weight, and variation along its length. As I mentioned yesterday (and will discuss further below) we have traditionally assumed that our drawings are monotone: black on white, or blue on white, or white on blue. Weight is usually referred to in decimal millimeters: a line is 0.5mm wide, or 0.18mm, or whatever. The numbers used in the 80s were the standard thickness of leads sold for use in hand drafting; we can now use any number we want. It turns out that the freedom to use other widths means little, since decades of trial and error had produced a set of lead thicknesses that were about as close as you could get before the differences became indistinguishable.

There were many types of variation along the length of a line in hand drafting, and many more now with CAD. Here’s a small selection from the Army Corps, combined with weights:

The center and match lines are both short/long dashed lines, the phantom line is a double-short dashed line, and the hidden line is an ordinary dashed line. There are many other possible variants including dash-dot, triple short dash, “barbed wire” with small xs along the line, half-toned or patterned lines, and so on. Personally, I’m not so sure that the difference between 0.35 and 0.25mm is enough to visually distinguish existing from new. But the point of this table is not to convince me, it’s to provide a consistent visual language across the work of the Corps. At my first job, there was a longer table that included dash-dotted lines, triple short dashes, and a few others. Each had a specific meaning, but since those meanings were defined by that firm, every drawing had to have a legend explaining them. The current Old Structures drafting manual has nine line types specified.

So, what to change? Visually, the most important thing for us, for our type of work, is to distinguish three types of structure: existing to remain, existing to be removed, and new. I already know that we disagree with the Corps about that, as their manual states “For clarity in as-built drawings, plan views of demolition work shall not be combined with those for new work. Demolition plans shall not show features that have no contractual significance to the work in the construction contract.” Most of the demolition we have is “cut-out demolition,” which is to say that we are removing specific pieces of a structure that will otherwise remain as is. Even when we have separate demolition plans, they usually show two or all three of the types I mentioned. Varying line weight and type is the traditional method of showing this, but I feel like this is a good place to use color. Since, as I mentioned yesterday, we may have statutory requirements for black and white, and people may print our PDFs in black and white, we can’t only use color, but we can combine it with the other attributes. What if hidden lines were not just lightweight and dashed but also gray. Or if existing to be demolished was “barbed wire” and red. That seems like an improvement in readability.