The diagram is not of a broken-back curve, as there is a tangent section between the two curves. Norman has it right...
After doing a little google search, it seems that engineers use term to describe two curves separated by a small section of tangent. They seem to be of concern when dealing with supers. As a surveyor, I was taught that the term describes non-tangent curves per Wattles.
On a side note, does anyone know if there is a PDF version of Wattles? I seemed to have misplaced my copy, not that anything has ever been 'Lost' in the mess I call an office.
So far I've only been able to tell that a "broken back curve" is an undefined colloquialism.
Others have posted that Wattles states that it is a non-tangent curve. However when I read Jon Payne's post that he found a definition like the original post I checked my Route Surveying text book (Meyer and Gibson) and found the same. So, it seems we have multiple definitions.
Personally I don't see why anyone would use broken back curve to define a non-tangent curve. If both phrases have the same number of syllables, where is the economy?
Colloquiallisms in general, are meant to make you sound wise, confusing or both.
"Colloquiallisms in general, are meant to make you sound wise, confusing or both."
I think we have different definitions of the word. Colloquialisms are just informal substitutions, so yeah the term 'broken back curves' is one. But I don't think it's used is to make anyone sound wise. I've only heard the term used between surveyors in conversation. Anything else, I use the term non-tangent curve.
The more interesting part of this thread is how our engineering counterparts use the term in a significantly different manner.
Personally, I use the term "broken back" specifically in cases where compound or reverse curves are non-tangential at their common point.
Writing Legal Descriptions in Conjunction with Survey Boundary Control by Gurdon H. Wattles. 4.22 (i)
Same here.
As usual, it depends...
I have a PDF of Wattles - not sure where I got it...
@ Jon Payne
My professor used The combination of a short length of tangent between two circular curves is referred to as a broken-back curve. to describe a broken-back curve. The source of this definition is the Wyoming State Department of Transportation, Appendix D - Alignment and Superelevation.
Thanks Jim
It looks like the term has multiple definitions depending on who is using it. Somehow, I had never heard the engineering use until I started doing some research as a result of this thread. I found multiple incidents of the engineering definition technical documents. I have used it in more of informal setting when discussing non-tangent curves with other surveyors.
Thankfully, no one has yet has made mention of Brokeback Mountain.
I've always understood a broken-back curve to be two curves in the same direction, separated by a short tangent section. They were used in the embryonic days of road construction, but usually replaced later with a single circular curve which provided safer travel.
Some days I feel like a museum piece. Back in the days when we designed and ran curves in the field using transit and tape our party chief often used what he called broken back curves on a long curve where running the forward and back tangents of the entire curve was not the best way to do it. Maybe the main PI was inaccessible or it would have created a very short foresight or backsight for turning the interior PI angle. In such a case we would project the back tangent to a convenient location and do the same with the forward tangent coming back. We would then use the distance from the PC to the broken back PI and measure ahead that same distance to the center of the road ahead. That was the PT of the first curve and The PC of the next. We would proceed in the same manner until the entire curve was complete. Sometimes doing the design in the field required a person without a calculator just using tables to introduce a short length between curves so that they wouldn't overlap. Now of course none of this is necessary. Compound curves are very similar if not the same. If there is a difference it may be that in some cases the point between curves is also a small deflection angle (non tangent and not recommended) thereby creating a broken back between tangents. If there is an official definition it is abused and misused. I always thought of it as a series of curves and tangents for one longer curve. Back to my display case.
